Fixed #22028 -- Documented that database signals aren't emitted for apps that laack a models module.

Thanks un33k for the suggestion and donjpacheco for the draft patch.

docs/ref/applications.txt

@@ -194,6 +194,10 @@ Read-only attributes
     from 'django/contrib/admin/models.pyc'>``.
 
     It may be ``None`` if the application doesn't contain a ``models`` module.
+    Note that the database related signals such as
+    :data:`~django.db.models.signals.pre_migrate` and
+    :data:`~django.db.models.signals.post_migrate`
+    are only emitted for applications that have a ``models`` module.
 
 Methods
 -------

docs/ref/signals.txt

@@ -377,7 +377,7 @@ pre_migrate
    :module:
 
 Sent by the :djadmin:`migrate` command before it starts to install an
-application.
+application. It's not emitted for applications that lack a ``models`` module.
 
 Any handlers that listen to this signal need to be written in a particular
 place: a ``management`` module in one of your :setting:`INSTALLED_APPS`. If
@@ -469,7 +469,8 @@ post_migrate
    :module:
 
 Sent by the :djadmin:`migrate` command after it installs an application, and the
-:djadmin:`flush` command.
+:djadmin:`flush` command. It's not emitted for applications that lack a
+``models`` module.
 
 Any handlers that listen to this signal need to be written in a particular
 place: a ``management`` module in one of your :setting:`INSTALLED_APPS`. If