docs: fix some examples in man pages

The man-examples job hasn't been running for a few months to verify
that these compile, so some mistakes slipped in.

Ref: #14654

getinfo: add CURLINFO_POSTTRANSFER_TIME_T

Returns the time, in microseconds, from the start until the last byte is
sent by libcurl (i.e. the request is sent off).

Closes #14189