Rollup merge of rust-lang#65007 - BO41:keywords, r=nikomatsakis

Mention keyword closing policy

closes rust-lang#59233 / rust-lang#59233 (comment)

rewording suggestions welcome

> Also in the referenced issue, the commit number of the new commit
> that could close that issue is not really informative. The PR number itself appeared in the issue
> is more informative and concise.

@lzutao what do you mean with that? Is this fixed by the new "May be fixed by #XXXXX"?

CONTRIBUTING.md

@@ -128,6 +128,14 @@ the master branch to your feature branch.
 Also, please make sure that fixup commits are squashed into other related
 commits with meaningful commit messages.
 
+GitHub allows [closing issues using keywords][closing-keywords]. This feature
+should be used to keep the issue tracker tidy. However, it is generally preferred
+to put the "closes #123" text in the PR description rather than the issue commit;
+particularly during rebasing, citing the issue number in the commit can "spam"
+the issue in question.
+
+[closing-keywords]: https://help.github.com/en/articles/closing-issues-using-keywords
+
 Please make sure your pull request is in compliance with Rust's style
 guidelines by running