diff --git a/docs/contributing/phabricator.rst b/docs/contributing/phabricator.rst
--- a/docs/contributing/phabricator.rst
+++ b/docs/contributing/phabricator.rst
@@ -218,6 +218,8 @@
 w.r.t. the previous review request, free form.
 Your message will become the changelog entry in Differential
 for this new version of the diff.
+By default this is your commit message, you **should change it**,
+because it is not useful to repeat the diff's title in its own changelog.
 
 Differential only care about the code diff, and not about the commits
 or their order.