pgsql: Reclassify replication-related GUC variables as "master" and "st

Reclassify replication-related GUC variables as "master" and "standby".

Per discussion, this structure seems more understandable than what was
there before. Make config.sgml and postgresql.conf.sample agree.

In passing do a bit of editorial work on the variable descriptions.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/60a81ad13374265cf9aecdc72daecd0410936779

Modified Files
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doc/src/sgml/config.sgml | 115 ++++++++++++++----------
src/backend/utils/misc/guc.c | 32 ++++---
src/backend/utils/misc/postgresql.conf.sample | 23 ++++--
src/include/utils/guc_tables.h | 5 +-
4 files changed, 103 insertions(+), 72 deletions(-)