docs($injector): fix inaccuracy in $provide.service docs

Closes #12664
Closes #12665

src/auto/injector.js

@@ -491,8 +491,20 @@ function annotate(fn, strictDi, name) {
  *
  * Register a **service constructor**, which will be invoked with `new` to create the service
  * instance.
- * This is short for registering a service where its provider's `$get` property is the service
- * constructor function that will be used to instantiate the service instance.
+ * This is short for registering a service where its provider's `$get` property is a factory
+ * function that returns an instance instantiated by the injector from the service constructor
+ * function.
+ *
+ * Internally it looks a bit like this:
+ *
+ * ```
+ * {
+ *   $get: function() {
+ *     return $injector.instantiate(constructor);
+ *   }
+ * }
+ * ```
+ *
  *
  * You should use {@link auto.$provide#service $provide.service(class)} if you define your service
  * as a type/class.