DOC: config: mention that url_ip and url_port fail when passed a name

DOC: config: mention that url_ip and url_port fail when passed a name

url_ip and url_port both rely on a call to url2sa() which tries to parse
digits in the authority part. This means that neither is usable with a
host name. For example www.example.org:443 will not return an IP address
nor even port 443 since the IP address parsing fails early. It's worth
being said since it's not obvious.