feat(teams): document behaviour of 'Prevent teams from being a member of another team' option

Signed-off-by: Cristian Scheid <cristianscheid@gmail.com>

Author: cristianscheid

user_manual/groupware/contacts.rst

@@ -240,6 +240,10 @@ Team options
 
 Various self-explanatory options are available to configure a team, to manage invites and membership, visibility of the team, allowance of other team membership and password protection.
 
+**Prevent teams from being a member of another team**
+
+When this option is enabled, the team can no longer be directly added as a member of another team. However, this restriction only applies to new direct additions. Existing memberships are kept, and inherited memberships are still possible if this team belongs to a parent team that gets added elsewhere.
+
 Shared items
 ~~~~~~~~~~~~
 .. versionadded:: 5.5