Add tip about accessing removed services in the testing doc

Author: mmenozzi

testing.rst

@@ -560,6 +560,15 @@ allows fetching both public and all non-removed private services::
 For a list of services available in your application, use the ``debug:container``
 command.
 
+.. tip::
+
+    Keep in mind that, if a private service is never used as a dependency of another service in
+    your application, it is then removed from the container. So, if you try to access a private
+    service in a test through the special test container and that service isn't used elsewhere
+    you'll get a ``ServiceNotFoundException``. The solution, depending on the context, is to
+    define the service as explicitly ``public`` or to inject it where you'll need it so Symfony
+    doesn't remove it.
+
 .. tip::
 
     The special container that gives access to private services exists only in