tiny feature request: user roles explainer

[annamorphism]

When I make users on CantusDB I have to remember which roles/"groups" do what (" "editor" is only if I need them to proofread, and "project manager" is only if they are a Debra--right? And I should check indexer, too?").
On Fragmentarium (though I can't actually create users there!) there's just a nice tidy explainer on the user page of what the roles do, like so:

I think if this were, say, a mouseover on the groups under "add user", it would save my brain a certain amount of lookup time.

[dchiller]

Yes, I think we could also simplify now that we are in Django. Currently, everyone who is a "project manager" is also "staff" all of whom are also "superusers". So we essentially use them interchangeably (although they mean slightly different things -- for example, you have to be "staff" to have access to the Taxonomy Manager and create users). But it would be great to streamline our permissions checks and share what they all mean in some kind of explainer like this.