Explain design choices for this API vs. alternatives

[johannhof]

Discussed this with @npdoty

As part of a justification for why the group wants to move this API forward at this time, we should write up why we chose the "thin layer" design vs. more purpose built APIs, particularly referencing the adoption of potentially much worse existing alternatives.

This seems crucial to enable wider review of the API post-FPWD

cc @RByers

[simoneonofri]

@johannhof I agree, I wrote something in the draft PR for the Security Consideration sections, tltr analysis here