[3.0.x] Fixed #30597 -- Clarified how to unapply migrations.

Backport of 45554fd5c5 from master

docs/topics/migrations.txt

@@ -347,6 +347,30 @@ Note that this only works given two things:
   that your database doesn't match your models, you'll just get errors when
   migrations try to modify those tables.
 
+Reverting migrations
+====================
+
+Any migration can be reverted with :djadmin:`migrate` by using the number of
+previous migrations::
+
+    $ python manage.py migrate books 0002
+    Operations to perform:
+      Target specific migration: 0002_auto, from books
+    Running migrations:
+      Rendering model states... DONE
+      Unapplying books.0003_auto... OK
+
+If you want to revert all migrations applied for an app, use the name
+``zero``::
+
+    $ python manage.py migrate books zero
+    Operations to perform:
+      Unapply all migrations: books
+    Running migrations:
+      Rendering model states... DONE
+      Unapplying books.0002_auto... OK
+      Unapplying books.0001_initial... OK
+
 .. _historical-models:
 
 Historical models