Docs: renamed input's state to message

Readers might feel confused with the actual input's `value` attribute.

docs/09.2-form-input-binding-sugar.md

@@ -33,14 +33,14 @@ Here's a simple form example without using `ReactLink`:
 
 var NoLink = React.createClass({
   getInitialState: function() {
-    return {value: 'Hello!'};
+    return {message: 'Hello!'};
   },
   handleChange: function(event) {
-    this.setState({value: event.target.value});
+    this.setState({message: event.target.value});
   },
   render: function() {
-    var value = this.state.value;
-    return <input type="text" value={value} onChange={this.handleChange} />;
+    var message = this.state.message;
+    return <input type="text" value={message} onChange={this.handleChange} />;
   }
 });
 ```
@@ -53,10 +53,10 @@ This works really well and it's very clear how data is flowing, however with a l
 var WithLink = React.createClass({
   mixins: [React.addons.LinkedStateMixin],
   getInitialState: function() {
-    return {value: 'Hello!'};
+    return {message: 'Hello!'};
   },
   render: function() {
-    return <input type="text" valueLink={this.linkState('value')} />;
+    return <input type="text" valueLink={this.linkState('message')} />;
   }
 });
 ```
@@ -82,14 +82,14 @@ There are two sides to `ReactLink`: the place where you create the `ReactLink` i
 
 var WithoutMixin = React.createClass({
   getInitialState: function() {
-    return {value: 'Hello!'};
+    return {message: 'Hello!'};
   },
   handleChange: function(newValue) {
-    this.setState({value: newValue});
+    this.setState({message: newValue});
   },
   render: function() {
     var valueLink = {
-      value: this.state.value,
+      value: this.state.message,
       requestChange: this.handleChange
     };
     return <input type="text" valueLink={valueLink} />;
@@ -107,10 +107,10 @@ As you can see, `ReactLink` objects are very simple objects that just have a `va
 var WithoutLink = React.createClass({
   mixins: [React.addons.LinkedStateMixin],
   getInitialState: function() {
-    return {value: 'Hello!'};
+    return {message: 'Hello!'};
   },
   render: function() {
-    var valueLink = this.linkState('value');
+    var valueLink = this.linkState('message');
     var handleChange = function(e) {
       valueLink.requestChange(e.target.value);
     };