curl_multi_wait: reduce timeout if the multi handle wants to

If the multi handle's pending timeout is less than what is passed into
this function, it will now opt to use the shorter time anyway since it
is a very good hint that the handle wants to process something in a
shorter time than what otherwise would happen.

curl_multi_wait.3 was updated accordingly to clarify

This is the reason for bug #1224

Bug: http://curl.haxx.se/bug/view.cgi?id=1224
Reported-by: Andrii Moiseiev