log-forward "host" option may be confusing because we often mention the
host field when talking about syslog RFC3164 or RFC5424 messages, but
neither rfc actually define "host" field. In fact, everywhere we used
"host field" we actually meant "hostname field" as documented in RFC5424.
This was a language abuse on our side.
In this patch we replace "host" with "hostname" where relevant in the
documentation to prevent confusion.
Thanks to Nick Ramirez for having reported the issue.