Adopt convention for TODO comments

We have 45 TODOs in our spec as of this writing. We used to use XXX for similar purposes; we have 31 of those. These TODOs: can indicate a lot of things.

1. The specification an incomplete description of the current protocol.
2. The specification is incorrect description of the current protocol.
3. The specification is unclear.
4. Some part of the specification is obsolete, but not marked as such.
5. The specification is accurate, but the protocol should be changed.
6. We think that one or more of 1-5 might apply, but we aren't sure, and don't know which.

This can get out of hand, and make it hard to search for the things we'd like to change, or find out whether these changes are urgent.

It would be cool to adopt a convention more specific than TODO for indicating which of these situations a TODO describes. That way we could grep for e.g. "What needs triage" or "what needs a new proposal".

cc @Diziet