opts: Warn CURLOPT_TIMEOUT overrides when set after CURLOPT_TIMEOUT_MS

Change CURLOPT_TIMEOUT doc to warn that if CURLOPT_TIMEOUT and
CURLOPT_TIMEOUT_MS are both set whichever one is set last is the one
that will be used.

Prior to this change that behavior was only noted in the
CURLOPT_TIMEOUT_MS doc.