pgsql: Doc: improve documentation about inheritance.

Doc: improve documentation about inheritance.

Clarify documentation about inheritance of check constraints, in
particular mentioning the NO INHERIT option, which didn't exist when
this text was written.

Document that in an inherited query, the applicable row security policies
are those of the explicitly-named table, not its children. This is the
intended behavior (per off-list discussion with Stephen Frost), and there
are regression tests for it, but it wasn't documented anywhere user-facing
as far as I could find.

Do a bit of wordsmithing on the description of inherited access-privilege
checks.

Back-patch to 9.5 where RLS was added.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/afb10b3b298c1012908a7d0f491a6a852c64e38d

Modified Files
--------------
doc/src/sgml/ddl.sgml | 32 +++++++++++++++++++++-----------
1 file changed, 21 insertions(+), 11 deletions(-)