Improve documentation around Container masks.

They are stackable in WebGL, but not Canvas.

Author: BenjaminDRichards

skills/groups-and-containers.md

@@ -40,7 +40,7 @@ fgLayer.add(this.add.sprite(400, 300, 'player'));
 | **Physics** | Via physics.add.group() | Limited (offsets if not at 0,0) | No |
 | **Input** | No (children can) | Yes (needs hit area shape) | No |
 | **Object pooling** | Yes (getFirstDead, kill) | No | No |
-| **Masks** | No | Yes (not per-child) | Yes |
+| **Masks** | No | Yes (not per-child in Canvas) | Yes |
 | **Alpha/blend/visible** | No (batch via setVisible) | Yes | Yes |
 | **Nesting** | N/A | Container in Container | Cannot go in Container |
 | **Extends** | EventEmitter | GameObject | List |
@@ -324,7 +324,7 @@ Cannot be added to a Container. Containers can be added to Layers.
 
 5. **Physics + Container is problematic.** If a Container is not at (0,0), physics bodies on children will be offset. Avoid physics bodies on Container children.
 
-6. **Container children cannot be individually masked.** Only the Container itself can have a mask. Masks do not stack for nested Containers.
+6. **Container children cannot be individually masked in Canvas rendering.** Only the Container itself can have a mask. Masks do not stack for nested Containers. Masks do stack in WebGL rendering.
 
 7. **Group.get() vs Group.getFirst() differ.** `get(x, y)` is shorthand for `getFirst(false, true, x, y)` -- finds first *inactive* member and creates if none found. `getFirst(state)` defaults to `active===false` without auto-creating.
 

src/gameobjects/container/Container.js

@@ -40,8 +40,10 @@ var tempTransformMatrix = new Components.TransformMatrix();
  *
  * Containers can include other Containers for deeply nested transforms.
  *
- * Containers can have masks set on them and can be used as a mask too. However, Container children cannot be masked.
- * The masks do not 'stack up'. Only a Container on the root of the display list will use its mask.
+ * Containers can have masks set on them and can be used as a mask too.
+ * Because masks are filters, the container's children can also have masks,
+ * and the Container's mask will be applied over the top.
+ * In Canvas rendering, only the Container's mask will be applied.
  *
  * Containers can be enabled for input. Because they do not have a texture you need to provide a shape for them
  * to use as their hit area. Container children can also be enabled for input, independent of the Container.