Add extra information for the imageData property.

Author: yahiko00

docs/Phaser.BitmapData.html

@@ -1886,7 +1886,8 @@ <h3 class="subsection-title">Members</h3>
 
 
     <div class="description">
-        <p>The context image data.</p>
+        <p>The context image data.
+Please note that a call to BitmapData.draw() or BitmapData.copy() does not update this property for performance reason. Use BitmapData.update() to do so.</p>
     </div>