[Meta] Proof-read and modernize spec

[rakuco]

This has been discussed in passing at TPAC in the last couple of years, so I'm finally filing an issue to keep track of this task.

Most of the spec text was written some 6-7 years ago, and since then new best practices have come up (for example, the Privacy Considerations and Security Considerations sections are expected to be separate these days). I also think the spec is not great at defining some terms and concepts, and some algorithms are a bit hand-wavy.

Doing an internal spec review and addressing these items would also show that this WG is committed to following the latest best practices and that this spec is well-maintained, in addition to helping us if we ever need to ask for a TAG review again.