[motion] <angle> notation should be defined as bearing angles

[fantasai]

The description for <angle> values right now is very confusingly written. It should just reference bearing angles, like gradients do.

Current prose:

The path is a line segment that starts from the position of the element and proceeds in the direction defined by the specified `` from the y axis. For ``, the initial value none is interpreted as 0deg, the element can be positioned along the y axis. If the `` value increases in the positive direction, the element moves clockwise.

Proposed prose:

The path is a line segment that starts from the position of the element and proceeds in the direction defined by the specified ``. As with [CSS gradients](https://www.w3.org/TR/css3-images/#gradients), `` values are interpreted as bearing angles, with `0deg` pointing up and positive angles representing clockwise rotation.