CIP-0010 | Add label-in-use query, clean up update process

[rphair]

@katomm offered a great suggestion to help find unused label numbers in #1169 (comment).

I've inserted that with some different wrapper language & removed some old language that was grammatically incorrect & maybe no longer meaningful today:

• all Reserved labels through 15 — not just 0 and 1 — are currently in use;
• the "machine-readable" registry.json file is also human readable, so the old term isn't meaningful;
• instead of "open your pull request" I've filled out that information a bit more with what editors have been requiring for these PRs.

(modified section, rendered from branch)

[Ryun1]

nice addition