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
------
REL8_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/d7a8e9a70ed3eaa854562dbfaf70fab78491c66c

Modified Files
--------------
doc/src/sgml/backup.sgml |   13 ++++++++++---
1 files changed, 10 insertions(+), 3 deletions(-)

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