- avoid writing "set ..." or "enable/disable ..." or "specify ..."
*All* options for curl_easy_setopt() are about setting or enabling
things and most of the existing options didn't use that way of
description.
- start with lowercase letter, unless abbreviation. For consistency.
- Some additional touch-ups
Closes#7688
The CURLOPT_SSH_PUBLIC_KEYFILE option has been documented to handle
empty strings specially since curl-7_25_0-31-g05a443a but the behavior
was unintentionally removed in curl-7_38_0-47-gfa7d04f.
This commit restores the original behavior and clarifies it in the
documentation that NULL and "" have both the same meaning when passed
to CURLOPT_SSH_PUBLIC_KEYFILE.
Bug: http://curl.haxx.se/mail/lib-2016-01/0072.html
