Add note about only 20 commits being included in push payload

doc/user/project/integrations/webhooks.md

@@ -54,6 +54,12 @@ Below are described the supported events.
 
 Triggered when you push to the repository except when pushing tags.
 
+> **Note:** When more than 20 commits are pushed at once, the `commits` web hook 
+  attribute will only contain the first 20 for performance reasons. Loading 
+  detailed commit data is expensive. Note that despite only 20 commits being 
+  present in the `commits` attribute, the `total_commits_count` attribute will
+  contain the actual total. 
+
 **Request header**:
 
 ```