Some animation doc improvements

[mweatherley]

Objective

Animation docs could use some clarification regarding:

• how exactly curves are evaluated
• how additive blend nodes actually work

Solution

Add some documentation that explains how curve domains are used and how additive blend nodes treat their children.

Commentary

The way additive blend nodes work right now is a little bit weird, since their first child's weight is ignored. Arguably this makes sense, since additive animations are authored differently from ordinary animations, but it also feels a bit strange. We could make the first node's weight actually be applied, and the present behavior would be recovered when the weight is set to 1.

The main disadvantage of how things are set up now is that combining a bunch of additive animations without a base pose is pretty awkward (e.g. to add them onto a base pose later in the graph). If we changed it, the main downside would be that reusing the same animation on different parts of the graph is harder; on the other hand, the weights can be locally reassigned by using blend nodes with no other children, which rectifies this shortfall.