ST_Summary needs to document output

[mwtoews]

Flags shown between '[' and ']' from ST_Summary are not documented.

E.g. here is an example with [ZMBS] and nested [ZM] flags:

mydb=# select st_summary('SRID=4326;MULTILINESTRING((0 0 0 0, 1 1 1 1),(2 2 2 2, 3 3 3 3))');
           st_summary
---------------------------------
                                +
 MultiLine[ZMBS] with 2 elements+
   Line[ZM] with 2 points       +
   Line[ZM] with 2 points       +

(1 row)

[mwtoews]

Also, &Z_support; for ST_Summary in doc/reference_accessor.xml isn't needed, since the output is not a geometry and doesn't really apply.

[strk]

Done in r8868 and r8869