Add entries for recent changes, including noting the JSONB format change
and the recent timezone data changes.  We should remove those two items
before 9.4 final: the JSONB change will be of no interest in the long
run, and it's not normally our habit to mention timezone updates in
major-release notes.  But it seems important to document them temporarily
for beta testers.

I failed to resist the temptation to wordsmith a couple of existing
entries, too.