document GV_FORMAT_POSTGIS

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@55371 15284696-431f-4ddb-bdfa-cd5b030d7da7

include/vect/dig_structs.h

@@ -1231,6 +1231,7 @@ struct Map_info
       - GV_FORMAT_NATIVE
       - GV_FORMAT_OGR
       - GV_FORMAT_OGR_DIRECT
+      - GV_FORMAT_POSTGIS
     */
     int format;
     /*!