Theme JSON: Clarify status of fixed position opt-in in appearance tools

[andrewserong]

What?

Fixes: #48635

Add a comment to the Theme JSON class clarifying the current status of the position.fixed opt-in within appearance tools.

Why?

As raised in #48635, without a comment, it isn't clear why there's a discrepancy between the core code and the Gutenberg code for this particular opt-in.

As discussed in #48635 (comment) and WordPress/wordpress-develop#3973 (comment), Gutenberg should be slightly more permissive than core here, so that it's easier to test subsequent explorations in adding fixed to the Group block, but this wasn't desired in core. In core, fixed is a valid setting, but it needs to be opted-in intentionally so that it isn't accidentally exposed anywhere.

How?

Add a comment to the Theme JSON class explaining the current status of the position.fixed opt-in. This uses the same // BEGIN EXPERIMENTAL and // END EXPERIMENTAL comments I've seen used elsewhere.

Testing Instructions

1. Proofread the comments.
2. (Optional) Load up the site editor or post editor with TT3 theme, and check that you can still access the Position panel and set Sticky (this just checks that I didn't add a typo or anything to break the PHP file)

[aaronrobertshaw]

Thanks for adding the detailed comment @andrewserong 👍

I think it helps explain why position.fixed is being added to the appearance tools opt-in within Gutenberg only and clearly marks its inclusion as experimental. There's one small typo in it though.

The sticky position support continues to function for me as well.

[andrewserong]

Thanks for the review and for spotting the typo @aaronrobertshaw!

[andrewserong]

Thanks for the reviews!