Fix description for "username" in User resource

[Xirado]

Usernames are unique across regular users.

[Lulalaby]

They aren't fully unique yet because bots still use discriminators. So the docs are currently still correct.

[Xirado]

Ahhh, totally forgot about Bots. yeah that makes sense. Maybe that should be included in the docs?

[Lulalaby]

I don't think this PR makes sense at the moment. Unless as a note with something like "Usernames are unique, except for bot users". But that sounds / feels unneeded at the moment imo.

[JustinBeckwith]

While I appreciate the sentiment, I think the way it's documented now is safer and more clear. If you need to account for bot users, which folks probably should .. they are not currently unique.