Adding short note on multiple users' usage

[giovannipizzi]

This will be extended in the future when better multiuser support will be available

Fixes #4005

[sphuber]

Thanks @giovannipizzi . The way you describe it here, we are essentially saying that we don't support multi-user configurations. Having each user install their own instance does not count. If that is what we want to do, I think we should make this more explicit. Something like the following as a first sentence. "Setups with multiple users for a single AiiDA instance is currently not supported".

That being said, even though we don't have a formal multi-user system in place, there are certain ways to have some kind of multiple user setups. I feel it might still be useful to note these here, albeit with a fat disclaimer of the relevant caveats. Would you agree?

[giovannipizzi]

I have accepted the suggestion, and removed the note on UUIDs vs PKs.
Instead, as discussed, for now I will not expand on ways to have multiple user-setup as I never tested them, or some of the other options might not guarantee enough security. We can always extend this section later, if we want.

This is ready to be reviewed again (and merged - I have rebased).

[sphuber]

Thanks @giovannipizzi some minor changes and then I will merge this

[csadorf]

This looks fine and I agree with @sphuber 's comments. I'll approve once those comments are addressed and the merge conflict is resolved.

[giovannipizzi]

Should be done now

[csadorf]

@sphuber Please merge at your discretion.