pgsql: Improve pg_autovacuum documentation to clarify that the enabled

Log Message:
-----------
Improve pg_autovacuum documentation to clarify that the enabled field cannot
prevent anti-wraparound vacuuming, and to caution against setting unreasonably
small values of freeze_max_age.  Also put in a notice that this catalog is
likely to disappear entirely in some future release.  Per discussion of
bug #3898 from Steven Flatt.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        catalogs.sgml (r2.160 -> r2.161)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml?r1=2.160&r2=2.161)