doc: Clarify and move "hygienic commit" note

The prior version mentions to update the tests *first*, which is wrong.
It must be updated at the same time in the same commit.

CONTRIBUTING.md

@@ -115,6 +115,7 @@ fixes or code moves with actual code changes.
 
 Make sure each individual commit is hygienic: that it builds successfully on its
 own without warnings, errors, regressions, or test failures.
+This means tests must be updated in the same commit that changes the behavior.
 
 Commit messages should be verbose by default consisting of a short subject line
 (50 chars max), a blank line and detailed explanatory text as separate