... between curl_multi_perform and curl_multi_wait/poll documentation
The `curl_multi_perform` documentation uses integer contextual
conversion to `bool` to test the resulting `CURLMcode`, while other
functions like `curl_multi_wait` and `curl_multi_poll` test against
`CURLM_OK`. (I was initially confused by this as it looked like some
docs call curl_multi_wait/poll on error, while some called only on
success. But that was my misread, not a docs problem.)
Also fixed the example to print which function call failed; previously
an error reported by `curl_multi_perform` was printed as a failure of
`curl_multi_wait`.
Closes#20444
af508e3641