Add optional normative guidance on bidi/whitespace serialization

[aphillips]

In response to merging #884 and the proposal in the bidi-usability design document, we agreed in the 2024-09-16 call to add an "optionally normative" section of the spec in the post-46 timeframe.

[aphillips]

Moving to 48

[aphillips]

I intended to implement this today, but note that, since we created the issue, we made the default bidi strategy normatively required. That makes this item obsolete.

[eemeli]

I don't think this is obsolete. We still ought to say when and what isolation or marks are to be used, as there are multiple options and situations where it's not clear whether isolation is to be preferred.