Update wording in Geospatial spec for CRS (#560)

Document existing CRS usage.

Author: milastdbx

Geospatial.md

@@ -44,8 +44,12 @@ locations on Earth.
 The default CRS `OGC:CRS84` means that the geospatial features must be stored
 in the order of longitude/latitude based on the WGS84 datum.
 
-Custom CRS can be specified by a string value. It is recommended to use an
-identifier-based approach like [Spatial reference identifier][srid].
+Non-default CRS values are specified by any string that uniquely identifies a coordinate reference system associated with this type.
+To maximize interoperability, suggested (but not limited to) formats for CRS are:
+* `<projjson>` - A complete CRS definition embedded directly using the [PROJJSON](https://proj.org/en/stable/specifications/projjson.html) specification. Example for `OGC:CRS83`: `{"$schema": "https://proj.org/schemas/v0.7/projjson.schema.json","type": "GeographicCRS","name": "NAD83 (CRS83)","datum": {"type": "GeodeticReferenceFrame"...`
+* `<authority>:<code>` - where `<authority>` represents some well known authorities, and `code` is the code used by the authority to identify the CRS. Examples are - `OGC:CRS84`, `OGC:CRS83`, `OGC:CRS27`, `EPSG:4326`, `EPSG:3857`, `IGNF:ATI`. See [https://spatialreference.org/](https://spatialreference.org/) for definitions of coordinate reference systems provided by some well known authorities.
+* `srid:<identifier>` -  A reference using a [Spatial reference identifier (SRID)](https://en.wikipedia.org/wiki/Spatial_reference_system#Identifier), where <identifier> is the numeric SRID value. For example: `SRID:0`. 
+* `projjson:<key_name>` - where <key_name> refers to a key within the file key-value metadata, where CRS definition in [PROJJSON](https://proj.org/en/stable/specifications/projjson.html) format is stored.
 
 For geographic CRS, longitudes are bound by [-180, 180] and latitudes are bound
 by [-90, 90].
@@ -150,18 +154,6 @@ In addition, the following rules are applied:
 [geometry-types]: https://github.com/opengeospatial/geoparquet/blob/v1.1.0/format-specs/geoparquet.md?plain=1#L159
 [wkb-integer-code]: https://en.wikipedia.org/wiki/Well-known_text_representation_of_geometry#Well-known_binary
 
-# CRS Customization
-
-CRS is represented as a string value. Writer and reader implementations are
-responsible for serializing and deserializing the CRS, respectively.
-
-As a convention to maximize the interoperability, custom CRS values can be
-specified by a string of the format `type:identifier`, where `type` is one of
-the following values:
-
-* `srid`: [Spatial reference identifier](https://en.wikipedia.org/wiki/Spatial_reference_system#Identifier), `identifier` is the SRID itself.
-* `projjson`: [PROJJSON](https://proj.org/en/stable/specifications/projjson.html), `identifier` is the name of a table property or a file property where the projjson string is stored.
-
 # Coordinate axis order
 
 The axis order of the coordinates in WKB and bounding box stored in Parquet