pgsql: Adjust documentation with respect to "unknown" timezone setting.

Adjust documentation with respect to "unknown" timezone setting.

The recent cleanup of GUC assign hooks got rid of the kludge of using
"unknown" as a magic value for timezone and log_timezone. But I forgot
to update the documentation to match, as noted by Martin Pitt.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1453cd8f82139a3e751c617dc6635f7b1c7abe14

Modified Files
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doc/src/sgml/config.sgml | 13 ++++++-------
src/backend/utils/misc/postgresql.conf.sample | 6 ++----
2 files changed, 8 insertions(+), 11 deletions(-)