- `Msg` or `msg` is now the canonical term for the substructure in a
room. It includes a `chat` subtype and (in the future) other
administration subtypes like member joining or leaving.
- `Message` or `message` can used in human oriented context like docs
and comments, but only when it is unambiguous.
- `message` is not chosen in code because it's hard to type (at least
for me!), and have ambiguous meaning of:
- "Human readable text" in context of `ApiError`'s field.
- "A unit of data transfer, datagram" in context of WebSocket Message.
- `item` is not chosen because it is overly generic.
- If a user is not a room member, HTTP 404 code=not_found will be
returned. This also happen for posting into not-joined public rooms,
and it can be interpreted as "room member is not found".
- If a user is a member but lacks the member permission to perform an
action, HTTP 403 code=permission_denied will be returned.
