pgsql: Document pg_tablespace.spcoptions.

Log Message:
-----------
Document pg_tablespace.spcoptions.

This should have been part of the original tablespace-options patch, but I
overlooked the need to update this portion of the documentation.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        catalogs.sgml (r2.216 -> r2.217)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml?r1=2.216&r2=2.217)