Add links to docs for addons and top level API (#6555)

This makes it easier to figure out where the docs live.

Googling for e.g. `react-addons-update` also works, but this should
make things easier for people that hyperclick directly to the source.

src/addons/ReactComponentWithPureRenderMixin.js

@@ -36,6 +36,8 @@ var shallowCompare = require('shallowCompare');
  * complex data structures this mixin may have false-negatives for deeper
  * differences. Only mixin to components which have simple props and state, or
  * use `forceUpdate()` when you know deep data structures have changed.
+ *
+ * See https://facebook.github.io/react/docs/pure-render-mixin.html
  */
 var ReactComponentWithPureRenderMixin = {
   shouldComponentUpdate: function(nextProps, nextState) {

src/addons/ReactFragment.js

@@ -31,8 +31,11 @@ var numericPropertyRegex = /^\d+$/;
 var warnedAboutNumeric = false;
 
 var ReactFragment = {
-  // Wrap a keyed object in an opaque proxy that warns you if you access any
-  // of its properties.
+  /**
+   * Wrap a keyed object in an opaque proxy that warns you if you access any
+   * of its properties.
+   * See https://facebook.github.io/react/docs/create-fragment.html
+   */
   create: function(object) {
     if (typeof object !== 'object' || !object || Array.isArray(object)) {
       warning(

src/addons/link/LinkedStateMixin.js

@@ -16,6 +16,7 @@ var ReactStateSetters = require('ReactStateSetters');
 
 /**
  * A simple mixin around ReactLink.forState().
+ * See https://facebook.github.io/react/docs/two-way-binding-helpers.html
  */
 var LinkedStateMixin = {
   /**

src/addons/link/ReactLink.js

@@ -37,6 +37,9 @@
 var React = require('React');
 
 /**
+ * Deprecated: An an easy way to express two-way binding with React. 
+ * See https://facebook.github.io/react/docs/two-way-binding-helpers.html
+ *
  * @param {*} value current value of the link
  * @param {function} requestChange callback to request a change
  */

src/addons/shallowCompare.js

@@ -16,6 +16,7 @@ var shallowEqual = require('shallowEqual');
 /**
  * Does a shallow comparison for props and state.
  * See ReactComponentWithPureRenderMixin
+ * See also https://facebook.github.io/react/docs/shallow-compare.html
  */
 function shallowCompare(instance, nextProps, nextState) {
   return (

src/addons/transitions/ReactCSSTransitionGroup.js

@@ -41,6 +41,11 @@ function createTransitionTimeoutPropValidator(transitionType) {
   };
 }
 
+/**
+ * An easy way to perform CSS transitions and animations when a React component
+ * enters or leaves the DOM.
+ * See https://facebook.github.io/react/docs/animation.html#high-level-api-reactcsstransitiongroup
+ */
 var ReactCSSTransitionGroup = React.createClass({
   displayName: 'ReactCSSTransitionGroup',
 

src/addons/transitions/ReactTransitionGroup.js

@@ -16,6 +16,11 @@ var ReactTransitionChildMapping = require('ReactTransitionChildMapping');
 
 var emptyFunction = require('emptyFunction');
 
+/**
+ * A basis for animatins. When children are declaratively added or removed,
+ * special lifecycle hooks are called.
+ * See https://facebook.github.io/react/docs/animation.html#low-level-api-reacttransitiongroup
+ */
 var ReactTransitionGroup = React.createClass({
   displayName: 'ReactTransitionGroup',
 

src/addons/update.js

@@ -66,6 +66,10 @@ function invariantArrayCase(value, spec, command) {
   );
 }
 
+/**
+ * Returns a updated shallow copy of an object without mutating the original.
+ * See https://facebook.github.io/react/docs/update.html for details.
+ */
 function update(value, spec) {
   invariant(
     typeof spec === 'object',