Add strftime escapes to Log config

All,

This page:

http://www.postgresql.org/docs/8.3/static/runtime-config-logging.html#RUNTIME-CONFIG-LOGGING-WHERE

has the following text:

The value is treated as a strftime pattern, so %-escapes can be used to
specify time-varying file names.

However, nowhere in our docs do we list sample strftime escapes. This
means that PG DBAs who are not programmers (and even some who are) end
up googling for them all the time. So I would like to add the following
text:

Useful excape codes which are available on most platforms include: %Y =
year, %m = month, %d = day of month, %H = hour, %M = minute, %a = short
weekday name, and %b = short month name.

--
-- Josh Berkus
PostgreSQL Experts Inc.
http://www.pgexperts.com