Rollup merge of #140773 - aDotInTheVoid:rdj-triagdfsadgs, r=dtolnay

triagebot: Better message for changes to `tests/rustdoc-json`

Followup to #140689 / #140606

Adds a message to changes to `tests/rustdoc-json`, instead of just pinging me. Hopefully this makes the significance of these tests clearer to people who otherwise wouldn't have context, so hopefully we can avoid that happening again. It's annoyingly hard to know how well this works, because the real test is seeing if it doesn't get ignored.

Predrag has [kindly offered](rust-lang/rust#140689 (comment)) to also get pinged here.

cc ``@jyn514`` ``@obi1kenobi``

r? ``@GuillaumeGomez``

Author: matthiaskrgr