If the `speed_scale` is set to a negative value, the animation plays in reverse.
The second parameter of `play()` still reverses as before. if `speed_scale` and the second parameter of `play()` is true, the animation plays forward.
Also updates the documentation to better describe the pausing and playing behaviour.
This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
