Document layer.effect: MultiEffect{} on the MultiEffect docs page

The docs for MultiEffect don't mention that you can do layer.effect: MultiEffect{}. Just found that trick mentioned on a completely different page. It would have been very useful to know earlier.

The MultiEffect docs page is extemely under-written. It needs to talk about:

• Using layer.effect: MultiEffect{}
• When you can use the source directly vs when you should wrap it in ShaderEffectSource
• How MultiEffect creates a new visual item that you need to size and position and use instead of the source item. Most people would assume it applies an effect to the source item, not create a new item alongside the source.
• How you should use hideSource on MultiEffect rather than make the source item invisible.