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

Author: aDotInTheVoid

triagebot.toml

@@ -1001,7 +1001,15 @@ message = "This PR changes how GCC is built. Consider updating src/bootstrap/dow
 message = "This PR changes a file inside `tests/crashes`. If a crash was fixed, please move into the corresponding `ui` subdir and add 'Fixes #<issueNr>' to the PR description to autoclose the issue upon merge."
 
 [mentions."tests/rustdoc-json"]
-cc = ["@aDotInTheVoid"]
+message = """
+These commits modify `test/rustdoc-json`.
+rustdoc-json is a **public** (but unstable) interface.
+
+Please ensure that if you've changed the output:
+- It's intentional.
+- The `FORMAT_VERSION` in `src/librustdoc-json-types` is bumped if necessary.
+"""
+cc = ["@aDotInTheVoid", "@obi1kenobi"]
 
 [mentions."tests/ui/deriving/deriving-all-codegen.stdout"]
 message = "Changes to the code generated for builtin derived traits."