CURLOPT_MAXCONNECTS.3: Reworked the description to be less confusing

...and corrected a related typo in curl_easy_setopt.3.
opts: Use common wording for TLS user/password option names

...and revised the proxy wording a little as well.
opts: Use common wording for MAIL related names
CURLOPT_SASL_IR.3: Added supported mechanism information

...and removed duplication of what protocols are supported from the
description text.
CURLOPT_XOAUTH2_BEARER.3: Corrected the OAuth version number