Added experimental jsdoc tag

[ggetz]

As mentioned in #4665, @pjcozzi

Adding the tag plus a description like so:

*
* @experimental This feature is using part of the 3D Tiles spec that is not final and is subject to change without Cesium's standard deprecation policy.
*

Results in:

[cesium-concierge]

@ggetz, thanks for the pull request! Maintainers, we have a signed CLA from @ggetz, 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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[pjcozzi]

LGTM.

Please update CHANGES.md and https://github.com/AnalyticalGraphicsInc/cesium/tree/master/Documentation/Contributors/CodingGuide#deprecation-and-breaking-changes to explain that a very small subset of APIs are experimental to, for example, allow for implementing draft formats and may have breaking changes without deprecation in future Cesium releases.

Anyone else want to look at this?

[ggetz]

Updated.

[pjcozzi]

Thanks!