docs($rootScope): correct the $watch listener parameter description

Closes #8322

Author: dbwhddn10

src/ng/rootScope.js

@@ -315,13 +315,12 @@ function $RootScopeProvider(){
        *
        *    - `string`: Evaluated as {@link guide/expression expression}
        *    - `function(scope)`: called with current `scope` as a parameter.
-       * @param {function()=} listener Callback called whenever the return value of
-       *   the `watchExpression` changes.
-       *
-       *    - `string`: Evaluated as {@link guide/expression expression}
-       *    - `function(newValue, oldValue, scope)`: called with current and previous values as
-       *      parameters.
+       * @param {function(newVal, oldVal, scope)} listener Callback called whenever the value
+       *    of `watchExpression` changes.
        *
+       *    - `newVal` contains the current value of the `watchExpression`
+       *    - `oldVal` contains the previous value of the `watchExpression`
+       *    - `scope` refers to the current scope
        * @param {boolean=} objectEquality Compare for object equality using {@link angular.equals} instead of
        *     comparing for reference equality.
        * @param {function()=} deregisterNotifier Function to call when the deregistration function