[Docs] Add reasons for using social accounts

docs/admin/04_socialaccounts.rst

@@ -4,6 +4,10 @@ Social accounts
 AlekSIS can authenticate users against third party applications using OAuth2
 or OpenID.
 
+This can be used to grant access to persons whose credentials shall not be
+managed in AlekSIS itself, for example because another authentication provider
+is already used throughout the school, or for guardians that can or should for
+some reason not get an LDAP account, or similar situations.
 
 .. warning::
   Social accounts are **not** working with two factor authentication! If a user