Update memoryWarning event description to mention it is iOS only (facebook#4972)

Co-authored-by: Bartosz Kaszubowski <gosimek@gmail.com>

Author: chetstone

docs/appstate.md

@@ -82,9 +82,9 @@ This example will only ever appear to say "Current state is: active" because the
 
 This event is received when the app state has changed. The listener is called with one of [the current app state values](appstate#app-states).
 
-### `memoryWarning`
+### `memoryWarning` <div className="label ios">iOS</div>
 
-This event is used in the need of throwing memory warning or releasing it.
+Fires when the app receives a memory warning from the operating system.
 
 ### `focus` <div className="label android">Android</div>
 

website/versioned_docs/version-0.77/appstate.md

@@ -82,9 +82,9 @@ This example will only ever appear to say "Current state is: active" because the
 
 This event is received when the app state has changed. The listener is called with one of [the current app state values](appstate#app-states).
 
-### `memoryWarning`
+### `memoryWarning` <div className="label ios">iOS</div>
 
-This event is used in the need of throwing memory warning or releasing it.
+Fires when the app receives a memory warning from the operating system.
 
 ### `focus` <div className="label android">Android</div>
 

website/versioned_docs/version-0.78/appstate.md

@@ -82,9 +82,9 @@ This example will only ever appear to say "Current state is: active" because the
 
 This event is received when the app state has changed. The listener is called with one of [the current app state values](appstate#app-states).
 
-### `memoryWarning`
+### `memoryWarning` <div className="label ios">iOS</div>
 
-This event is used in the need of throwing memory warning or releasing it.
+Fires when the app receives a memory warning from the operating system.
 
 ### `focus` <div className="label android">Android</div>
 

website/versioned_docs/version-0.79/appstate.md

@@ -82,9 +82,9 @@ This example will only ever appear to say "Current state is: active" because the
 
 This event is received when the app state has changed. The listener is called with one of [the current app state values](appstate#app-states).
 
-### `memoryWarning`
+### `memoryWarning` <div className="label ios">iOS</div>
 
-This event is used in the need of throwing memory warning or releasing it.
+Fires when the app receives a memory warning from the operating system.
 
 ### `focus` <div className="label android">Android</div>
 

website/versioned_docs/version-0.80/appstate.md

@@ -82,9 +82,9 @@ This example will only ever appear to say "Current state is: active" because the
 
 This event is received when the app state has changed. The listener is called with one of [the current app state values](appstate#app-states).
 
-### `memoryWarning`
+### `memoryWarning` <div className="label ios">iOS</div>
 
-This event is used in the need of throwing memory warning or releasing it.
+Fires when the app receives a memory warning from the operating system.
 
 ### `focus` <div className="label android">Android</div>
 

website/versioned_docs/version-0.81/appstate.md

@@ -82,9 +82,9 @@ This example will only ever appear to say "Current state is: active" because the
 
 This event is received when the app state has changed. The listener is called with one of [the current app state values](appstate#app-states).
 
-### `memoryWarning`
+### `memoryWarning` <div className="label ios">iOS</div>
 
-This event is used in the need of throwing memory warning or releasing it.
+Fires when the app receives a memory warning from the operating system.
 
 ### `focus` <div className="label android">Android</div>
 

website/versioned_docs/version-0.82/appstate.md

@@ -82,9 +82,9 @@ This example will only ever appear to say "Current state is: active" because the
 
 This event is received when the app state has changed. The listener is called with one of [the current app state values](appstate#app-states).
 
-### `memoryWarning`
+### `memoryWarning` <div className="label ios">iOS</div>
 
-This event is used in the need of throwing memory warning or releasing it.
+Fires when the app receives a memory warning from the operating system.
 
 ### `focus` <div className="label android">Android</div>
 

website/versioned_docs/version-0.83/appstate.md

@@ -82,9 +82,9 @@ This example will only ever appear to say "Current state is: active" because the
 
 This event is received when the app state has changed. The listener is called with one of [the current app state values](appstate#app-states).
 
-### `memoryWarning`
+### `memoryWarning` <div className="label ios">iOS</div>
 
-This event is used in the need of throwing memory warning or releasing it.
+Fires when the app receives a memory warning from the operating system.
 
 ### `focus` <div className="label android">Android</div>