CURLOPT_CAINFO.3: polished wording

Clarify the functionality when built to use Schannel and Secure
Transport and stop calling it the "recommended" or "preferred" way and
instead rather call it the default.

Removed the reference to the ssl comparison table as it isn't necessary.

Reported-by: Richard Alcock
Bug: https://curl.haxx.se/mail/lib-2019-06/0019.html
Closes #4005