pgsql: Improve 9.5 release notes.

Improve 9.5 release notes.

1) Add sgml comments referencing commits. This is useful to search for
missing items etc.

The comments containing the commit notes are an excerpt from:
git log --date=short \
--pretty='format:%cd [%h] %<(8,trunc)%cN: %<(48,trunc)%s%n%n%w(,4,4)%b%n' \
$(git merge-base origin/master upstream/REL9_4_STABLE)..origin/master
2) Improve a handful of existing notes
3) Add missing entries about a couple features.
4) Add a bunch of straight-forward FIXMEs

Branch
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master

Details
-------
http://git.postgresql.org/pg/commitdiff/c0b0501925eacbf2d9c10cd231bf8a14e7c9ef4c

Modified Files
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doc/src/sgml/release-9.5.sgml | 688 ++++++++++++++++++++++++++++++++++++++---
1 file changed, 651 insertions(+), 37 deletions(-)