pgsql: Improve documentation around logging_collector and use of stderr

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
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/e076ee6d3f1bf9ece52eb7c9dacea8f91e4c991c

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