docs($rootScope.Scope): improve clarity describing $watch with no lis…

…tener

The previous explanation in parentheses created a bit of confusion because the documentation stated to leave off the `listener`, but then said "be prepared for multiple calls to your listener". The new explanation clarifies that it is indeed the `watchExpression` that will be executed multiple times.

Closes angular#12429

src/ng/rootScope.js

@@ -276,9 +276,9 @@ function $RootScopeProvider() {
        *
        *
        * If you want to be notified whenever {@link ng.$rootScope.Scope#$digest $digest} is called,
-       * you can register a `watchExpression` function with no `listener`. (Since `watchExpression`
-       * can execute multiple times per {@link ng.$rootScope.Scope#$digest $digest} cycle when a
-       * change is detected, be prepared for multiple calls to your listener.)
+       * you can register a `watchExpression` function with no `listener`. (Be prepared for
+       * multiple calls to your `watchExpression` because it will execute multiple times in a
+       * single {@link ng.$rootScope.Scope#$digest $digest} cycle if a change is detected.)
        *
        * After a watcher is registered with the scope, the `listener` fn is called asynchronously
        * (via {@link ng.$rootScope.Scope#$evalAsync $evalAsync}) to initialize the