Improve instructions about using git notes

Notable record the commands that can be used to recover from a conflict
with the server version of the notes (merging notes is more involved and
usually not worth it, they can just be re-added after resetting).

docs/changes.txt

@@ -182,9 +182,28 @@ NOTE: This file is updated only before the release, please use
 
     $ git log --notes=changelog --format='%N' v3.1.4..|grep .
 
-      to see all the change log entries since the last release and use
+      to see all the change log entries since the last release.
 
-    $ git notes --ref=changelog add -m 'wxPort: description.' to update it.
+      To update the notes, fetch them first:
+
+    $ git fetch origin refs/notes/changelog:refs/notes/changelog
+
+      then use the following command to update them locally
+
+    $ git notes --ref=changelog add -m 'wxPort: description.'
+
+      and finally push it to the server.
+
+    $ git push origin refs/notes/changelog:refs/notes/changelog
+
+      If this fails due to a conflict because you had forgotten to
+      run git-fetch first, you can always reset your local notes
+      (LOSING YOUR CHANGES TO THEM, so make sure to make a copy)
+
+    $ git fetch origin refs/notes/changelog
+    $ git update-ref refs/notes/changelog FETCH_HEAD
+
+      and then redo "git-notes add" and git-push.
 
 
 3.1.4: (released 2020-07-22)