Document and refactor the CanonicalIdResolver extension point.

The extension point has been introduced in https://github.com/jenkinsci/jenkins/commit/af42534defea4503347dfc0ec442fadf51cf3336
It does not follow common extension point practices (e.g. “all()” methods) and has no Javadoc.

So I decided to polish it a bit. The core also adds some TODOs for review