Clarify subgroup permissions

What does this MR do?

Clarify documentation on a user's effective permission in a subgroup.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/51519

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides