Update docs: PerInstanceColorAppearance vertex format and other primitive API stuff

[nmschulte-aviture]

I believe FLAT_VERTEX_FORMAT's documentation is supposed to read "when true", not "false".

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As well, the documentation for VERTEX_FORMAT mis-matches the code; I've included a documentation change to make them match.

See also #8045; a bug moved from this PR.

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CLA is signed and CONTRIBUTORS.md is already updated:
https://github.com/Aviture/cesium/blame/fix-perinstancecolorappearance-doc/CONTRIBUTORS.md#L69

[cesium-concierge]

Thank you so much for the pull request @nmschulte-aviture! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

The Pull Request Guidelines is a handy reference for making sure your PR gets accepted quickly, so make sure to skim that.

• ❌ Missing CONTRIBUTORS.md entry.
  • Please add yourself to the contributors file!
• ❌ Missing CLA.
  • Please send in a Contributor License Agreement (CLA) and comment back here to let us know to check this!
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[cesium-concierge]

Thanks again for your contribution @nmschulte-aviture!

No one has commented on this pull request in 30 days. Maintainers, can you review, merge or close to keep things tidy?

I'm going to re-bump this in 30 days. If you'd like me to stop, just comment with @cesium-concierge stop. If you want me to start again, just delete the comment.

[cesium-concierge]

Thanks again for your contribution @nmschulte-aviture!

No one has commented on this pull request in 30 days. Maintainers, can you review, merge or close to keep things tidy?

I'm going to re-bump this in 30 days. If you'd like me to stop, just comment with @cesium-concierge stop. If you want me to start again, just delete the comment.

[hpinkos]

Thanks @nmschulte-aviture!

[cesium-concierge]

Thanks for the awesome work @nmschulte-aviture! Your contribution is about to launch to millions of users with the next release. 🚀

Do you mind if we tweet about it? CC @OmarShehata @slchow.