docs(on-comment): warn against built-in commands

Add a note advising users not to use PAC's standard GitOps
commands (/test, /retest, /cancel, /ok-to-test) as on-comment
annotation patterns, since they are processed before on-comment
matching.

Signed-off-by: Akshay Pant <akpant@redhat.com>

Author: theakshaypant

docs/content/docs/guides/event-matching/comment-and-label.md

@@ -36,6 +36,8 @@ The `on-comment` annotation follows the pull_request [Policy]({{< relref "/docs/
 
 {{< callout type="info" >}}
 The `on-comment` annotation works with pull_request events. For push events, Pipelines-as-Code supports it only [when targeting the main branch without arguments]({{< relref "/docs/guides/gitops-commands/push-commands" >}}).
+
+Avoid using Pipelines-as-Code's built-in [GitOps commands]({{< relref "/docs/guides/gitops-commands" >}}) — such as `/test`, `/retest`, `/cancel`, or `/ok-to-test` — as `on-comment` patterns. These commands are processed before `on-comment` matching and can lead to unexpected behavior.
 {{< /callout >}}
 
 ## Matching on pull request labels