[Sync-EN] Document curl_errno() and curl_error() behavior with multi handles - #1770

Syncs reference/curl/functions/curl-errno.xml and reference/curl/functions/curl-error.xml with php/doc-en#5233.

• curl_errno(): adds the note that a handle executed through curl_multi_exec() always reports 0, the per-handle error code being the result key returned by curl_multi_info_read().
• curl_error(): adds the matching note, the empty string being returned, and points at passing that result key to curl_strerror().
• The see-also lists gain curl_multi_info_read(), and curl_strerror() on the curl_error() page.

EN-Revision bumped to ff7a0054b19ab80290b365d1417550807f82755a on both files.

Fixes: #1689