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.

doc/developer-notes.md

@@ -805,9 +805,6 @@ General Bitcoin Core
      on the master branch. Otherwise, all new pull requests will start failing the tests, resulting in
      confusion and mayhem.
 
-  - *Explanation*: If the test suite is to be updated for a change, this has to
-    be done first.
-
 Logging
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