pgsql: Disallow the combination VACUUM FULL FREEZE for safety's sake,

Log Message:
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Disallow the combination VACUUM FULL FREEZE for safety's sake, for the
reasons I outlined in pghackers a few days ago.

Also, undo someone's overly optimistic decision to reduce tuple state
checks from if (...) elog() to Asserts. If I trusted this code more,
I might think it was a good idea to disable these checks in production
installations. But I don't.

Modified Files:
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pgsql/doc/src/sgml:
manage-ag.sgml (r2.36 -> r2.37)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/manage-ag.sgml.diff?r1=2.36&r2=2.37)
pgsql/doc/src/sgml/ref:
vacuum.sgml (r1.35 -> r1.36)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/vacuum.sgml.diff?r1=1.35&r2=1.36)
pgsql/src/backend/commands:
vacuum.c (r1.296 -> r1.297)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.296&r2=1.297)