Explainer: Expand alternatives considered section

[RByers]

In Chromium's Intent To Experiment, @slightlyoff says:

It would be good to see a more thorough considered alternatives section in the explainer. It's not immediately clear what (in code) the alternatives would entail.

Alex, you mean for the different proposals for this API that have been considered and linked to (not the existing alternatives like QR code scanning), right? They each have their own little explainer that is linked (back from when we were debating all the options and trying to converge). I wouldn't want to suck all that complexity into this explainer but perhaps I could summarize and compare/contrast a bit to provide context for someone who wants to click through to read the details of each discarded alternative?

A bunch of the alternatives are really just different spellings of the same thing (eg. should it be on navigator.credential and unified more with FedCM, or should it be it's own credential container on navigator.identity?). But one big class of differences which might be worth highlighting more in the explainer is those which fix the protocol and format to one the browser understands (like Apple's original proposal) and those which are intended to be more generic and extensible (as we've come to now). There's a pretty interesting and still hotly-debated tension here which I could try to summarize in the explainer if you think it would be useful (though it might involve a lot of wordsmithing and PR debate to align on something both camps feel is accurate).