pgsql: Update docs as to when WAL logging can be skipped.

Log Message:
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Update docs as to when WAL logging can be skipped.

In 8.4 and prior, WAL-logging could potentially be skipped whenever
archive_mode=off.  With streaming replication, this is now true only
if max_wal_senders=0.

Fujii Masao, with light copyediting by me

Modified Files:
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    pgsql/doc/src/sgml:
        backup.sgml (r2.151 -> r2.152)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml?r1=2.151&r2=2.152)
        perform.sgml (r1.75 -> r1.76)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/perform.sgml?r1=1.75&r2=1.76)