pgsql: Update pg_resetxlog's documentation on multixacts

Update pg_resetxlog's documentation on multixacts

I added some more functionality to it in 0ac5ad5134f27 but neglected to
add it to the docs.

Per Peter Eisentraut in message
1367112171.32604.4.camel@vanquo.pezone.net

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/9db4ad44eb02326bbe8eeaf7d1dd452844764a20

Modified Files
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doc/src/sgml/ref/pg_resetxlog.sgml  |   22 +++++++++++++---------
src/bin/pg_resetxlog/pg_resetxlog.c |    2 +-
2 files changed, 14 insertions(+), 10 deletions(-)