Federation: Document the Host header; Clarify how literal IP addresses are handled
Federation: Document the Host header; Clarify how literal IP addresses are handled

Fixes https://github.com/matrix-org/matrix-doc/issues/1161

The IP address clarification is to add an explicit mention of how to handle the case. The prior documentation assumed that all servers would be resolvable with DNS, and does technically have a fallback to use the fallback port, however making it clear feels like a good idea.
matrix-org/matrix-doc,matrix-org/matrix-doc,matrix-org/matrix-doc,matrix-org/matrix-doc