add accessibility to NumberControl through sliderOptions, added doc, #…

…452

js/NumberControl.js

@@ -4,6 +4,9 @@
  * Control for changing a Property of type {number}.
  * Consists of a labeled value, slider and arrow buttons.
  *
+ * Number Control provides accessible content exclusively through the slider, please pass accessibility related
+ * customizations through options to the slider.
+ *
  * @author Chris Malley (PixelZoom, Inc.)
  */
 define( require => {
@@ -169,7 +172,8 @@ define( require => {
     // a11y - for alternative input, the number control is accessed entirely through slider interaction and these
     // arrow buttons are not tab navigable
     assert && assert( options.arrowButtonOptions.tagName === undefined,
-      'NumberControl handles alternative input for arrow buttons' );
+      'NumberControl\'s accessible content is just the slider, do not set accessible content on the buttons. Instead ' +
+      'set a11y through options.sliderOptions.' );
     options.arrowButtonOptions.tagName = null;
 
     // Slider options for track (if not specified as trackNode)
@@ -269,7 +273,7 @@ define( require => {
     };
     numberProperty.link( arrowEnabledListener );
 
-    // @public {HSlider} - for access to AccessibleValueHandler API
+    // @public {HSlider} - for access to accessibility API
     this.slider = new HSlider( numberProperty, numberRange, options.sliderOptions );
 
     // major ticks

js/demo/ComponentsScreenView.js

@@ -577,9 +577,9 @@ define( function( require ) {
         font: new PhetFont( 14 )
       },
       sliderOptions: {
-        trackSize: new Dimension2( 150, 3 )
-      },
-      containerTagName: 'div'
+        trackSize: new Dimension2( 150, 3 ),
+        containerTagName: 'div'
+      }
     };
     demoParent.addChild( new VBox( {
       resize: false, // Don't readjust the size when the slider knob moves all the way to the right