diff --git a/doc/requirements.txt b/doc/requirements.txt
index fada2eb09..91ff9ceac 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.
 sphinx>=1.6.2 # BSD
 openstackdocstheme>=1.17.0 # Apache-2.0
 reno>=2.5.0 # Apache-2.0