Revert "Delete contrib/xml2's legacy implementation of xml_is_well_formed()."
Revert "Delete contrib/xml2's legacy implementation of xml_is_well_formed()."

This partly reverts commit 20540710e83f2873707c284a0c0693f0b57156c4.
Since we've given up on adding PGDLLEXPORT markers to PG_FUNCTION_INFO_V1,
there's no need to remove the legacy compatibility function.  I kept the
documentation changes, though, as they seem appropriate anyway.

50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb