Added --noinput in migrate commmand's docs.

docs/ref/django-admin.txt

@@ -780,6 +780,15 @@ Allows creating tables for apps without migrations. While this isn't
 recommended, the migrations framework is sometimes too slow on large projects
 with hundreds of models.
 
+.. django-admin-option:: --noinput, --no-input
+
+Suppresses all user prompts. An example prompt is asking about removing stale
+content types.
+
+.. versionchanged:: 1.9
+
+    The ``--no-input`` alias was added.
+
 ``runserver``
 -------------