pgsql: Documentation improvement and minor code cleanups for the latch

Documentation improvement and minor code cleanups for the latch facility.

Improve the documentation around weak-memory-ordering risks, and do a pass
of general editorialization on the comments in the latch code. Make the
Windows latch code more like the Unix latch code where feasible; in
particular provide the same Assert checks in both implementations.
Fix poorly-placed WaitLatch call in syncrep.c.

This patch resolves, for the moment, concerns around weak-memory-ordering
bugs in latch-related code: we have documented the restrictions and checked
that existing calls meet them. In 9.2 I hope that we will install suitable
memory barrier instructions in SetLatch/ResetLatch, so that their callers
don't need to be quite so careful.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6760a4d4029121981bf3ec24847ddfbacecc070d

Modified Files
--------------
src/backend/port/unix_latch.c | 149 ++++++++++++++++++-------------------
src/backend/port/win32_latch.c | 25 +++---
src/backend/replication/syncrep.c | 20 +++--
src/backend/storage/lmgr/proc.c | 2 +-
src/include/storage/latch.h | 69 ++++++++++++++++-
5 files changed, 163 insertions(+), 102 deletions(-)