Doc/maintainer subgroup subtleties

What does this MR do?

As per the discussion on !29718, better documentation is needed to make clear that:

1. Maintainers in existing groups cannot create subgroups, but maintainers in new groups can create subgroups
2. The setting can be changed for any group
3. The setting can be changed by admins in the admin area
4. The setting can be changed by owners in the group settings (this needs to be verified)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation

@hiroponz, @ifarkas, @eread