Clarify subgroup permissions

doc/user/group/subgroups/index.md

@@ -140,8 +140,10 @@ From the image above, we can deduct the following things:
 You need to be an Owner of a group in order to be able to add members to it.
 
 To override a user's membership of an ancestor group (the first group they were
-added to), simply add the user in the new subgroup again, but with different
-permissions.
+added to), simply add the user in the new subgroup again, but with a higher set of
+permissions. Note that a user's permissions in a subgroup cannot be lower
+than in any of its ancestor groups and, hence, you cannot reduce a user's permissions
+in a subgroup with respect to its ancestor groups.
 
 For example, if User0 was first added to group `group-1/group-1-1` with Developer
 permissions, then they will inherit those permissions in every other subgroup