[PostGIS] #4764: Doc update for ST_AsGeoJSON

[PostGIS]

#4764: Doc update for ST_AsGeoJSON
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  Reporter:  geozelot       |      Owner:  robe
      Type:  patch          |     Status:  new
  Priority:  medium         |  Milestone:
 Component:  documentation  |    Version:  master
Resolution:                 |   Keywords:  docs
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Comment (by strk):

 Regina: I found this ticket referenced in a github PR and came looking.
 It's been sitting here for 3 years now. Today it doesn't fully apply but
 one part of it does, using "geom_column" instead of "geomcolumnname" and
 giving default value, so I wonder: shouldn't these issues be intercepted
 by the garden test ?
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/4764#comment:2>
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