pgsql: Fix relcache for policies, and doc updates

Fix relcache for policies, and doc updates

Andres pointed out that there was an extra ';' in equalPolicies, which
made me realize that my prior testing with CLOBBER_CACHE_ALWAYS was
insufficient (it didn't always catch the issue, just most of the time).
Thanks to that, a different issue was discovered, specifically in
equalRSDescs. This change corrects eqaulRSDescs to return 'true' once
all policies have been confirmed logically identical. After stepping
through both functions to ensure correct behavior, I ran this for
about 12 hours of CLOBBER_CACHE_ALWAYS runs of the regression tests
with no failures.

In addition, correct a few typos in the documentation which were pointed
out by Thom Brown (thanks!) and improve the policy documentation further
by adding a flushed out usage example based on a unix passwd file.

Lastly, clean up a few comments in the regression tests and pg_dump.h.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/ff27fcfa0affe16405e801ed55fed10e7bc75216

Modified Files
--------------
doc/src/sgml/ddl.sgml | 111 ++++++++++++++++++++++++++++-
doc/src/sgml/ref/alter_policy.sgml | 2 +-
doc/src/sgml/ref/create_policy.sgml | 6 +-
src/backend/utils/cache/relcache.c | 4 +-
src/bin/pg_dump/pg_dump.h | 2 +-
src/test/regress/expected/rowsecurity.out | 10 +--
src/test/regress/sql/rowsecurity.sql | 10 +--
7 files changed, 127 insertions(+), 18 deletions(-)