[3.1.x] Fixed #32273 -- Doc'd AdminSite.unregister().

Backport of bebd4cfa8f from master

docs/ref/contrib/admin/index.txt

@@ -2947,6 +2947,13 @@ Templates can override or extend base admin templates as described in
     abstract. and ``django.contrib.admin.sites.AlreadyRegistered`` if a model
     is already registered.
 
+.. method:: AdminSite.unregister(model_or_iterable)
+
+    Unregisters the given model class (or iterable of classes).
+
+    Raises ``django.contrib.admin.sites.NotRegistered`` if a model isn't
+    already registered.
+
 .. _hooking-adminsite-to-urlconf:
 
 Hooking ``AdminSite`` instances into your URLconf

docs/spelling_wordlist

@@ -747,6 +747,7 @@ unpickled
 unpickling
 unpopulated
 unpythonic
+Unregisters
 unrendered
 unreproducible
 Unreviewed