docs: PATCH /guild/id/vanity-url isn't documented

[vladfrangu]

Mainly, this would be useful to know the result of a call to this endpoint, as well as the params that are meant to be sent. (although the only param required is code)

EDIT: I don't know if this is meant to be documented in the first place. Probably not. But I don't know

[msciotti]

I think this one may be better left undocumented. Vanity URLs do not often change once settled on, and if so, it should be done by the partner who owns the server, which probably means doing it via the server settings UI.

I understand that bots with certain permissions could change it, but I think I'd rather leave this undocumented.

It's no problem at all to suggest these things. Sometimes (like the GET route) we just overlook certain endpoints. Sometimes, like this case (or login endpoints), we have reasons not to.