Fixed #14165 - Documented that TransactionMiddleware only applies to the default database.

docs/ref/middleware.txt

@@ -203,9 +203,9 @@ Transaction middleware
 
 .. class:: TransactionMiddleware
 
-Binds commit and rollback to the request/response phase. If a view function
-runs successfully, a commit is done. If it fails with an exception, a rollback
-is done.
+Binds commit and rollback of the default database to the request/response
+phase. If a view function runs successfully, a commit is done. If it fails with
+an exception, a rollback is done.
 
 The order of this middleware in the stack is important: middleware modules
 running outside of it run with commit-on-save - the default Django behavior.