[WebAPI] Update animation.reverse() and the setter of playbackRate to match the spec

> [!IMPORTANT]
> This issue will be worked on by MDN's core team of writers. If you want to contribute, please see the [contributor board](https://github.com/orgs/mdn/projects/25) for issues where we're accepting pull requests.

## Acceptance criteria

- [ ] The listed features are documented sufficiently on MDN
- [ ] BCD is updated
- [ ] Interactive example and data repos are updated if appropriate
- [ ] The content has been reviewed as needed

### For folks helping with Firefox related documentation

> [!NOTE]
> Release notes are now generated at the beginning of the cycle, so a review is all that should be needed for the task. Obviously if there is a gap, fill it.

- [ ] Set bugs to `dev-doc-complete`
- [ ] Review/add entry to Firefox release notes for enabled/preview features
- [ ] Add/remove entry to Firefox experimental features page for preference/released features

## Features to document

Docs are probably already inline with spec - but worth a check

## Related Gecko bugs

- [Bug 2046973](https://bugzil.la/2046973) — Update animation.reverse() and the setter of playbackRate to match the spec

## Other

- [ ] Check content open issues to see if any pertain to the subject matter. If there are any that can be closed because of the work, do so. If there are any that can be fixed relatively quickly because of the knowledge from completing this issue and you have time, feel free to go ahead and fix them.

Start by locating the MDN content covering animation.reverse() and the playbackRate setter, then compare it with the related Gecko bug and the applicable specification behavior. Check whether BCD, interactive examples, data repositories, and Firefox release-note or preference pages need updates; done means the documentation is reviewed and all applicable acceptance criteria are complete.

Written by the indexing model from the issue text.