Context

This MR does some light clean up in the protocol, in particular validate.ml. It also updates and extends the documentation.

The MR should be easy to review commit-by-commit. Indeed, all changes are obvious refactoring, renaming, or comment-only changes.

Checklist

Document the interface of any function added or modified (see the coding guidelines)
[no change] Document any change to the user interface, including configuration parameters (see node configuration)
[no new functionality] Provide automatic testing (see the testing guide).
[N/A] For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
Select suitable reviewers using the Reviewers field below.
Select as Assignee the next person who should take action on that MR