Clarify that custom roles can be applied to subgroups

What does this MR do and why?

• There was a question from Support about whether custom roles can be applied to subgroup memberships.
• They can! Also to project memberships. But the custom role itself must be defined at the root group level.
• This distincton is a little challenging to understand and was not reflected in our docs very well. This MR is my attempt at improvement.

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