Update AnimFrame docs with info about when paint happens.

[xStrom]

The AnimFrame docs were still from an era before we had partial invalidation. Nowadays an AnimFrame event does not necessarily lead to a paint, because paint is guarded by a check for invalid regions existing.

I also fixed a few rustdoc warnings around doc formating.

[xStrom]

Alright I rewrote the docs with a more general focus and using common Druid terms like paint instead of talking about rendering which is a Druid internal term.

Does it look better now @jneem?