Add documentation for $Shape - Resolves #5246

[aikar]

No description provided.

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[MichaelDeBoey]

Thanks for this one @aikar! 🙂

[MichaelDeBoey]

I think this can be merged after the conflict is solved?

CC: @asolove @samwgoldman @mroch @nmote

[mvitousek]

It looks like #6694, which was merged last year, contained similar documentation for $Shape. Thank you for the contribution, and sorry we didn't get a chance to review and merge your diff; feel free to reopen if there's anything in your commit that didn't get included in the docs as they exist now which you'd like us to have a closer look at.