Clarify documentation about subgroup creation setting

Specify that group owners can change whether maintainers can create
subgroups in the settings of the group, and that administrators can
also change this setting in the admin area.

doc/user/group/subgroups/index.md

@@ -80,7 +80,10 @@ By default, groups created in:
 - GitLab 12.2 or later allow both Owners and Maintainers to create subgroups.
 - GitLab 12.1 or earlier only allow Owners to create subgroups.
 
-This setting can be for any group by an Owner or Administrator.
+The setting can be changed for any group by:
+
+- A group owner. Select the group, and navigate to **Settings > General > Permissions, LFS, 2FA**.
+- An administrator. Navigate to **Admin Area > Overview > Groups**, select the group, and choose **Edit**.
 
 For more information check the
 [permissions table](../../permissions.md#group-members-permissions). For a list