Fixed #28588 -- Doc'd User.has_perm() & co. behavior for active superusers.

Equivalent note for PermissionsMixin was added in d33864ed138f65070049a3ac20ee98e03a1442b9.

docs/ref/contrib/auth.txt

@@ -221,7 +221,8 @@ Methods
         Returns ``True`` if the user has the specified permission, where perm
         is in the format ``"<app label>.<permission codename>"``. (see
         documentation on :ref:`permissions <topic-authorization>`). If the user is
-        inactive, this method will always return ``False``.
+        inactive, this method will always return ``False``. For an active
+        superuser, this method will always return ``True``.
 
         If ``obj`` is passed in, this method won't check for a permission for
         the model, but for this specific object.
@@ -231,7 +232,8 @@ Methods
         Returns ``True`` if the user has each of the specified permissions,
         where each perm is in the format
         ``"<app label>.<permission codename>"``. If the user is inactive,
-        this method will always return ``False``.
+        this method will always return ``False``. For an active superuser, this
+        method will always return ``True``.
 
         If ``obj`` is passed in, this method won't check for permissions for
         the model, but for the specific object.
@@ -240,7 +242,8 @@ Methods
 
         Returns ``True`` if the user has any permissions in the given package
         (the Django app label). If the user is inactive, this method will
-        always return ``False``.
+        always return ``False``. For an active superuser, this method will
+        always return ``True``.
 
     .. method:: email_user(subject, message, from_email=None, **kwargs)