[css-fonts-4] Description of font-display: swap should better explain involved trade-offs

[johannhof]

https://drafts.csswg.org/css-fonts-4/#valdef-font-face-font-display-swap

As outlined in WICG/interventions#58 (and linked issues), usage of font-display: swap may be a trade-off that is applicable to a number of sites. General advice on the internet seems to be the opposite of what the spec describes, recommending to use swap for loading prominent text immediately, although it seems to have gotten more nuanced at this point. Overall I don't think the advice given in the spec currently is particularly applicable ("only use for small pieces of text"). It's also not entirely clear to me why a brand logo of all things would benefit from being displayed in a non-brand style.

Following up from the interventions issue it may make sense to alter the wording to explain the trade-off with FOUT and that using swap may be worthwhile for many sites.