[css-anchor-position-1] Improve accessibility guidance

[fantasai]

The description here:
https://drafts.csswg.org/css-anchor-position-1/#accessibility
spends several paragraphs saying 3 useful things:

• Anchor positioning doesn't affect non-visual UAs, so don't rely on it to create semantic connections. (No mention of e.g. keyboard navigation in visual UAs.)
• The “Popover API” (which is named but not linked, explained, or illustrated anywhere in this spec) can be used to perform semantic linking when that's appropriate.
• The aria-describedby and aria-details attributes might also help.

Other than that, there are no examples (of either good or bad practices), or concrete actionable guidance that would help an author actually do things correctly.

The goal of the section should be to help authors understand the implications of what they're doing, identify what tools they have available to do a good job, and explain when and how to use them. They should come away knowing how to think about the interaction of accessibility and anchor positioning, and be able to implement the guidance, at least for straightforward cases such as the basic use cases that drove the spec.

I don't think it's fulfilling that goal yet.