diff --git a/doc/requirements.txt b/doc/requirements.txt
index 26e9264e..303fd926 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,3 +1,6 @@
+# The order of packages is significant, because pip processes them in the order
+# of appearance. Changing the order has an impact on the overall integration
+# process, which may cause wedges in the gate later.
 # Notes:
 # reno needs openstackdocstheme which needs reno (cycle dep).
 # os-api-ref needs openstackdocstheme which needs os-api-ref (cycle dep).