Update 9.4 release notes for commits through today.

 Update 9.4 release notes for commits through today.
 
 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.