Spec size limits are misleading

Spec says `room_id` MUST NOT exceed 255 bytes; https://matrix.org/docs/spec/client_server/r0.6.1#size-limits

But the spec also says a server_name can be over 255 chars on its own https://matrix.org/docs/spec/appendices#server-name

(hostname 255 chars + `:` + port 5 digits) = 261

Given that a room ID must have a `!`, a localpart and a `:` it is actually impossible to use 255 dns-chars for the server name even with a hostname of 253 chars and no port identifier.

Start by comparing the linked size-limits section with the server-name appendix, including the stated maximum lengths and room ID structure. The work is done when the specification no longer presents contradictory limits and clearly explains the applicable maximum.

Written by the indexing model from the issue text.