Document flex ad mediaType / bidResponse options (prebid#6324)

* Document flex ad mediaType / bidResponse options

* Update Flex banner example with documentation link

Added a reference to Google request object documentation for Flex banner example.

---------

Co-authored-by: Patrick McCann <patmmccann@gmail.com>

Author: dgirardi

dev-docs/adunit-reference.md

@@ -84,7 +84,8 @@ See the table below for the list of properties in the `mediaTypes` object of the
 {: .table .table-bordered .table-striped }
 | Name | Scope | Type | Description |
 |---------+----------+---------------------------------------+-----------------------------------------------------------------------------------------|
-| `sizes` | Required | Array[Number] or Array[Array[Number]] | All sizes this ad unit can accept. Examples: `[400, 600]`, `[[300, 250], [300, 600]]`. Prebid recommends that the sizes auctioned by Prebid should be the same auctioned by AdX and GAM OpenBidding, which means AdUnit sizes should match the GPT sizes. |
+| `sizes` | Required if `format` is not provided | Array[Number] or Array[Array[Number]] | All sizes this ad unit can accept. Examples: `[400, 600]`, `[[300, 250], [300, 600]]`. Prebid recommends that the sizes auctioned by Prebid should be the same auctioned by AdX and GAM OpenBidding, which means AdUnit sizes should match the GPT sizes. |
+| `format` | Required if `sizes` is not provided | Array of ORTB [Format](https://github.com/InteractiveAdvertisingBureau/openrtb2.x/blob/main/2.6.md#objectformat) objects | Alternative to `sizes`, and takes precedence over it. Allows for more options, such as . | 
 | `pos` | Optional | Integer | OpenRTB page position value: 0=unknown, 1=above-the-fold, 3=below-the-fold, 4=header, 5=footer, 6=sidebar, 7=full-screen |
 | `name` | Optional | String | Name for this banner ad unit. Can be used for testing and debugging. |
 
@@ -222,6 +223,41 @@ pbjs.addAdUnits({
 });
 ```
 
+#### Flex banner example
+
+See the [Google request object documentation](https://developers.google.com/authorized-buyers/rtb/openrtb-guide#flexslot-object) for additional details. These fields were removed from OpenRTB 2.6 but remain popular. 
+
+```javascript
+pbjs.addAdUnits({
+   code: slot.code,
+   mediaTypes: {
+     banner: {
+       expdir: [1, 2, 3, 4],
+       format: [{w: 250, h: 250}],
+       wmin: 250,
+       wmax: 375,
+       hmin: 250,
+       hmax: 250,
+     },
+   },
+   ortb2Imp: {
+     banner: {
+       ext: {
+         flexslot: {
+           wmin: 250,
+           wmax: 375,
+           hmin: 250,
+           hmax: 250
+         }
+       }
+     }
+   },
+   bids: [
+     // ...
+   ]
+})
+```
+
 <a name="adUnit-video-example"></a>
 
 ### Video

dev-docs/bidder-adaptor.md

@@ -514,11 +514,13 @@ The parameters of the `bidResponse` object are:
 
 | Key          | Scope                                       | Description                                                                                                                                   | Example                              |
 |--------------+---------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------+--------------------------------------|
-| `requestId`  | Required                                    | The bid ID that was sent to `spec.buildRequests` as `bidRequests[].bidId`. Used to tie this bid back to the request.                          | 12345                                |
-| `cpm`        | Required                                    | The bid price. We recommend the most granular price a bidder can provide                                                                      | 3.5764                               |
-| `currency`   | Required                                    | 3-letter ISO 4217 code defining the currency of the bid.                                                                                      | `"EUR"`                              |
-| `width`      | Required                                    | The width of the returned creative. For video, this is the player width.                                                                      | 300                                  |
-| `height`     | Required                                    | The height of the returned creative. For video, this is the player height.                                                                    | 250                                  |
+| `requestId`  | Required                                       | The bid ID that was sent to `spec.buildRequests` as `bidRequests[].bidId`. Used to tie this bid back to the request.                          | 12345                                |
+| `cpm`        | Required                                       | The bid price. We recommend the most granular price a bidder can provide                                                                      | 3.5764                               |
+| `currency`   | Required                                       | 3-letter ISO 4217 code defining the currency of the bid.                                                                                      | `"EUR"`                              |
+| `width`      | Required except when using `wratio` / `hratio` | The width of the returned creative. For video, this is the player width.                                                                      | 300                                  |
+| `height`     | Required except when using `wratio` / `hratio` | The height of the returned creative. For video, this is the player height.                                                                    | 250                                  |
+| `wratio`     | Required for flex ads                          | For flexible banner only, the relative width of the creative                                                                                 | 6                                    |
+| `hratio`     | Required for flex ads                          | For flexible banner only, the relative height of the creative                                                                                | 5                                    |
 | `ad`         | Required                                    | The creative payload of the returned bid.                                                                                                     | `"<html><h3>I am an ad</h3></html>"` |
 | `ttl`        | Required                                    | Time-to-Live - how long (in seconds) Prebid can use this bid. See the [FAQ entry](/dev-docs/faq.html#does-prebidjs-cache-bids) for more info.   | 360                                  |
 | `creativeId` | Required                                    | A bidder-specific unique code that supports tracing the ad creative back to the source.                                                       | `"123abc"`                           |