curl_pushheader_byname/bynum.3: document in their own man pages

These two functions were added in 7.44.0 when CURLMOPT_PUSHFUNCTION was
introduced but always lived a life in the shadows, embedded in the
CURLMOPT_PUSHFUNCTION man page. Until now.

It makes better sense and gives more visibility to document them in
their own stand-alone man pages.

Closes #11286

.github/scripts/cleanspell.pl

@@ -69,6 +69,7 @@ while(<F>) {
         $_ =~ s/curl_mime_(subparts|addpart|filedata|data_cb)//g;
         $_ =~ s/curl_ws_(send|recv|meta)//g;
         $_ =~ s/curl_url_(dup)//g;
+        $_ =~ s/curl_pushheader_by(name|num)//g;
         $_ =~ s/libcurl-env//g;
         $_ =~ s/(^|\W)((tftp|https|http|ftp):\/\/[a-z0-9\-._~%:\/?\#\[\]\@!\$&'()*+,;=]+)//gi;
         print O $_;