diff --git a/doc/source/third_party.rst b/doc/source/third_party.rst
index 9ecc4a5ee8..f98162bc8b 100644
--- a/doc/source/third_party.rst
+++ b/doc/source/third_party.rst
@@ -27,6 +27,10 @@ Requirements
 
   * A system can also be set up to only do '+1' reviews and leave all the
     '-1's to be manually confirmed.
+
+* A third-party system may only leave one comment per patch set
+  (unless it is retriggered).
+
 * The maintainers are responsible for re-triggering tests when their third
   party testing system breaks.
 * Support recheck to request re-running a test.