Fixed #18577 - Clarified middleware initialization.

Thanks Lukasz Balcerzak for the patch.

docs/topics/http/middleware.txt

@@ -199,7 +199,8 @@ of caveats:
   define ``__init__`` as requiring any arguments.
 
 * Unlike the ``process_*`` methods which get called once per request,
-  ``__init__`` gets called only *once*, when the Web server starts up.
+  ``__init__`` gets called only *once*, when the Web server responds to the
+  first request.
 
 Marking middleware as unused
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~