Clarify callback parameters

Clarifies the callback parameters for React.Children.map()
and React.Children.forEach() by noting that they can also
contain an optional `index` parameter as a second
argument.

Author: blairwilcox

content/docs/reference-react.md

@@ -154,15 +154,15 @@ Verifies the object is a React element. Returns `true` or `false`.
 #### `React.Children.map`
 
 ```javascript
-React.Children.map(children, function[(thisArg)])
+React.Children.map(children, function(thisArg, [index]))
 ```
 
-Invokes a function on every immediate child contained within `children` with `this` set to `thisArg`. If `children` is a keyed fragment or array it will be traversed: the function will never be passed the container objects. If children is `null` or `undefined`, returns `null` or `undefined` rather than an array.
+Invokes a function on every immediate child contained within `children` with `this` set to `thisArg`. The optional `index` parameter refers to the index of the current child in the array. If `children` is a keyed fragment or array it will be traversed: the function will never be passed the container objects. If children is `null` or `undefined`, returns `null` or `undefined` rather than an array.
 
 #### `React.Children.forEach`
 
 ```javascript
-React.Children.forEach(children, function[(thisArg)])
+React.Children.forEach(children, function(thisArg, [index]))
 ```
 
 Like [`React.Children.map()`](#reactchildrenmap) but does not return an array.