pgsql: Remove warning about num_sync being too large in synchronous_sta

Remove warning about num_sync being too large in synchronous_standby_names.

If we're not going to reject such setups entirely, throwing a WARNING in
check_synchronous_standby_names() is unhelpful, because it will cause the
warning to be logged again every time the postmaster receives SIGHUP.
Per discussion, just remove the warning.

In passing, improve the documentation for synchronous_commit, which had not
gotten the word that now there can be more than one synchronous standby.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/17d5db352c1780f4721664f67bc3a3f3b1cf933c

Modified Files
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doc/src/sgml/config.sgml | 45 ++++++++++++++++++---------------------
src/backend/replication/syncrep.c | 45 ---------------------------------------
2 files changed, 21 insertions(+), 69 deletions(-)