Apply suggested wording

to better describe the situation

CONTRIBUTING.md

@@ -130,8 +130,10 @@ 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. But in pull requests only. Please
-do not add this to your commit message.
+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