pgsql: Doc: small improvements for documentation about VACUUM freezing.

Doc: small improvements for documentation about VACUUM freezing.

Mostly, explain how row xmin's used to be replaced by FrozenTransactionId
and no longer are.  Do a little copy-editing on the side.

Per discussion with Egor Rogov.  Back-patch to 9.4 where the behavioral
change occurred.

Discussion: <575D7955.6060209@postgrespro.ru>

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6cdc26e482182a872d10a899cc4bbdb11364720b

Modified Files
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doc/src/sgml/maintenance.sgml | 42 ++++++++++++++++++++++++++++++++----------
1 file changed, 32 insertions(+), 10 deletions(-)