Explain Service without selector is for non-Pod backends (#32602)

* Minor edit for clarity

The previous phrasing didn't emphasize the point that the reason you define a Service with no selectors is to point to a backend that's not a Pod, and the emphasis on the external nature of the backend

* Add note that Services w/o selectors, but +Endpoints is the technique to use for abstracting external backends

Co-authored-by: Rey Lejano <[email protected]>

Co-authored-by: Rey Lejano <[email protected]>

Author: timrosenblatt

content/en/docs/concepts/services-networking/service.md

@@ -158,9 +158,9 @@ Each port definition can have the same `protocol`, or a different one.
 
 ### Services without selectors
 
-Services most commonly abstract access to Kubernetes Pods, but they can also
-abstract other kinds of backends.
-For example:
+Services most commonly abstract access to Kubernetes Pods thanks to the selector,
+but when used with a corresponding Endpoints object and without a selector, the Service can abstract other kinds of backends, 
+including ones that run outside the cluster. For example:
 
 * You want to have an external database cluster in production, but in your
   test environment you use your own databases.