Streaming replication and document about avoiding WAL-logging

Hi,

In 9.0, some SQL commands can avoid WAL-logging only when
not only archive_mode is off but also max_wal_senders is
zero (new condition since 9.0). But I forgot modifying the
documents about this behavior enough :(

The attached patch adds the description about that new
condition for WAL-logging.

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center