docs($rootScope): added $eval locals parameter documentation

Added documentation for `locals` parameter of `$eval` method.

src/ng/rootScope.js

@@ -710,7 +710,8 @@ function $RootScopeProvider(){
        *
        *    - `string`: execute using the rules as defined in  {@link guide/expression expression}.
        *    - `function(scope)`: execute the function with the current `scope` parameter.
-       *
+       * 
+       * @param {(object)=} locals Local variables object, useful for overriding values in scope.
        * @returns {*} The result of evaluating the expression.
        */
       $eval: function(expr, locals) {