Improve documentation around logging_collector and use of stderr.

In backup.sgml, point out that you need to be using the logging collector
if you want to log messages from a failing archive_command script.  (This
is an oversimplification, in that it will work without the collector as
long as you're not sending postmaster stderr to /dev/null; but it seems
like a good idea to encourage use of the collector to avoid problems
with multiple processes concurrently scribbling on one file.)

In config.sgml, do some wordsmithing of logging_collector discussion.

Per bug #6518 from Janning Vygen

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/3f47e145f1869f147a807e5a2cb80d21a13e10ae

Modified Files
--------------
doc/src/sgml/backup.sgml |   13 ++++++++++---
doc/src/sgml/config.sgml |   33 ++++++++++++++++++++++++---------
2 files changed, 34 insertions(+), 12 deletions(-)

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