Change JSDoc to correctly document Cesium.Math.*

[emackey]

Fixes #6233.

[cesium-concierge]

@emackey, thanks for the pull request! Maintainers, we have a signed CLA from @emackey, so you can review this at any time.

⚠️ I noticed that CHANGES.md has not been updated. If this change updates the public API in any way, fixes a bug, or makes any non-trivial update, please add a bullet point to CHANGES.md and comment on this pull request so we know it was updated. For more info, see the Pull Request Guidelines.

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[hpinkos]

Thanks @emackey! I would make a comment in CHANGES.md since this was a doc error reported by our users

[emackey]

Done.

[hpinkos]

Thanks @emackey!