docs(testing): 'testingService' -> 'twainService' plus clarity (#2692)

Author: wardbell

public/docs/ts/latest/guide/testing.jade

@@ -743,11 +743,12 @@ a(href="#top").to-top Back to top
   ## _whenStable_
   The test must wait for the `getQuote` promise to resolve in the next turn of the JavaScript engine.
 
-  This test has no direct access to the promise returned by the call to `testService.getQuote` 
-  which is private and inaccessible inside `TwainComponent`.
+  This test has no direct access to the promise returned by the call to `twainService.getQuote` 
+  because it is buried inside `TwainComponent.ngOnInit` and therefore inaccessible to a test that
+  probes only the component API surface.
 
   Fortunately, the `getQuote` promise is accessible to the _async test zone_ 
-  which intercepts all promises issued within the _async_ method call.
+  which intercepts all promises issued within the _async_ method call _no matter where they occur_.
 
   The `ComponentFixture.whenStable` method returns its own promise which resolves when the `getQuote` promise completes.
   In fact, the _whenStable_ promise resolves when _all pending asynchronous activities within this test_ complete ... the definition of "stable".