Enhance documentation of the build-in standby mode, explaining the retry

loop in standby mode, trying to restore from archive, pg_xlog and
streaming.

Move sections around to make the high availability chapter more
coherent: the most prominent part is now a "Log-Shipping Standby Servers"
section that describes what a standby server is (like the old
"Warm Standby Servers for High Availability" section), and how to
set up a warm standby server, including streaming replication, using the
built-in standby mode. The pg_standby method is desribed in another
section called "Alternative method for log shipping", with the added
caveat that it doesn't work with streaming replication.