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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master

Details
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http://git.postgresql.org/pg/commitdiff/4e15a4db5e65e43271f8d20750d6500ab12632d0

Modified Files
--------------
src/backend/port/unix_latch.c     |  166 ++++++++++++++++++-------------------
src/backend/port/win32_latch.c    |   49 ++++++------
src/backend/replication/syncrep.c |   20 +++--
src/backend/storage/lmgr/proc.c   |    2 +-
src/include/storage/latch.h       |   70 +++++++++++++++-
5 files changed, 184 insertions(+), 123 deletions(-)