Add links to documentation for Mix, Compose and Fill

[PoignardAzur]

The concepts behind the Mix, Compose and Fill types are well-known in computer graphics, but they're not intuitive for a newcomer and hard to explain through text. (And even a experience developer might not remember which is which.)

We should probably edit their respective doc comments to add links to pages illustrating the underlying concepts. We can probably find good resources on MDN or Wikipedia.