Add a note to rustc-dev-guide

Author: Kobzol

Diff for: src/tests/ci.md

@@ -180,6 +180,8 @@ their results can be seen [here](https://github.com/rust-lang-ci/rust/actions),
 although usually you will be notified of the result by a comment made by bors on
 the corresponding PR.
 
+Note that if you start the default try job using `@bors try`, it will skip building several `dist` components and running post-optimization tests, to make the build duration shorter. If you want to execute the full build as it would happen before a merge, add an explicit `try-job` pattern with the name of the default try job (currently `dist-x86_64-linux`).
+
 Multiple try builds can execute concurrently across different PRs.
 
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