Merge pull request #2928 from OSGeo/backport-2926-to-8.2

[Backport 8.2] DOC: Add warning in proj_as_proj_string about potential information loss with CRS

src/iso19111/c_api.cpp

@@ -1589,6 +1589,11 @@ const char *proj_as_wkt(PJ_CONTEXT *ctx, const PJ *obj, PJ_WKT_TYPE type,
  * The returned string is valid while the input obj parameter is valid,
  * and until a next call to proj_as_proj_string() with the same input
  * object.
+ * 
+ * \warning If a CRS object was not created from a PROJ string, 
+ *          exporting to a PROJ string will in most cases
+ *          cause a loss of information. This can potentially lead to
+ *          erroneous transformations.
  *
  * This function calls
  * osgeo::proj::io::IPROJStringExportable::exportToPROJString().