docs:  mention tgconstrindid supports exclusion constraints

Dean Rasheed
Disallow use of SSL v3 protocol in the server as well as in libpq.

Commit 820f08cabdcbb8998050c3d4873e9619d6d8cba4 claimed to make the server
and libpq handle SSL protocol versions identically, but actually the server
was still accepting SSL v3 protocol while libpq wasn't.  Per discussion,
SSL v3 is obsolete, and there's no good reason to continue to accept it.
So make the code really equivalent on both sides.  The behavior now is
that we use the highest mutually-supported TLS protocol version.

Marko Kreen, some comment-smithing by me