[PostGIS] #5739: ST_AsGeoJSON docs should have an example of CRS / bbox

[PostGIS]

#5739: ST_AsGeoJSON docs should have an example of CRS / bbox
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 Reporter:  robe           |      Owner:  robe
     Type:  defect         |     Status:  new
 Priority:  medium         |  Milestone:  PostGIS 3.5.0
Component:  documentation  |    Version:  3.4.x
 Keywords:                 |
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 Mentioned in irc/matrix, and sounds like a good idea to me

 ```
 Hi, I'm trying to understand the GeoJSON implementation in Postgis.
 Specifically, the
 [documentation](https://postgis.net/docs/ST_AsGeoJSON.html) mentions that
 the `options` parameter allows you to include bbox and CRS in the output.
 However, the [spec](https://www.rfc-editor.org/rfc/rfc7946#section-3) only
 makes mention of the `bbox` field. This
 12:54 PM
 leaves me somewhat unclear on the field name used for CRS information
 (based on documentation alone), especially since the postgis documentation
 page does not include any examples where either bbox or crs is included.
 Do anyone know how these fields are encoded in the GeoJSON produced by
 `ST_AsGeoJson`?
 ```
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/5739>
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