Fixes:
> `mailbox` of this room set to `test@domain@domain`
Previously fixed in 97a4d6c7bc, but it seems like we unintentionally
reintroduced this bug again at some point after that.
When someone first joins a room, they see some commands (`mailbox`,
`owner`, ..) and they know they are getters and setters, but they have
no good example as to how to use them.
Is it `!pm mailbox SOMETHING` or `!pm mailbox=SOMETHING` or something
else?
It's better if the introduction text gives you the full command you need
to get started (e.g. `!pm mailbox SOME_MAILBOX`), instead of a partial
command that you don't know how to use (e.g. `!pm mailbox` - this is
merely a getter and will not set your mailbox to `SOME_MAILBOX`).
Starting from this, I thought it would be a good idea to make all
option getters tell you how the commands are to be used. If you send
`!pm mailbox` and it tells you "not yet set", it should also tell you
how to actually set it (e.g. `!pm mailbox VALUE`).
