Add documentation for triagebot [canonicalize-issue-links] handler

[Urgau]

Implemented in rust-lang/triagebot#1897.

Rendered

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[Urgau]

cc @Kobzol

[RalfJung]

This is great, thanks!

We may want to enable this in the main repo as well, or else pulling main repo commits over to Miri/clippy will reference the wrong issues there. They likely don't exist, so nothing gets closed incorrectly, but the links are still broken.

Also FWIW, bors-generated merge commits are a major source of such non-canonical links; I doubt this bot can do anything about that though.

[Kobzol]

New bors puts the whole repo name into the merge commit issue link. The problem is that the "Auto merge of #12345" message has been used for a lot of years and there is a bunch of tooling that depends on it and parses the format 😅 So we'll also need to change that.

[Urgau]

We may want to enable this in the main repo as well, or else pulling main repo commits over to Miri/clippy will reference the wrong issues there. They likely don't exist, so nothing gets closed incorrectly, but the links are still broken.

I didn't think of the other way around, but it makes sense issue links are a problem in both ways.

I'm currently deploying the feature on a few repos and will let it bake before enabling it on our main repo, but sure that I will do after making sure it works well enough and doesn't cause problem.