Update github-trigger-event-options.adoc (#9345)

* Update github-trigger-event-options.adoc

* address comments

* Update jekyll/_cci2/github-trigger-event-options.adoc

Co-authored-by: Rosie Yohannan <[email protected]>

---------

Co-authored-by: Rosie Yohannan <[email protected]>

Author: BeFunes

jekyll/_cci2/github-trigger-event-options.adoc

@@ -98,6 +98,15 @@ a| * `opened`
 
 |===
 
+All pipelines triggered by link:https://docs.github.com/en/webhooks/webhook-events-and-payloads#pull_request[pull request events] also have the following xref:variables/#pipeline-values[pipeline values] populated:
+
+* `pipeline.event.github.pull_request.base.ref`: the name of the *base* (or target) branch of the PR - that is the branch that will receive the changes.
+* `pipeline.event.github.pull_request.head.ref`: the name of the *head* branch of the PR - that is the branch containing the changes to be merged.
+* `pipeline.event.github.pull_request.draft`: a boolean value indicating whether the pull request is a draft.
+* `pipeline.event.github.pull_request.title`: the title of the pull request.
+* `pipeline.event.github.pull_request.number`: the numeric identifier of the pull request.
+
+These values are extracted directly from the GitHub event payload. The part after prefix `pipeline.event.github.*` matches the corresponding field in the pull request event structure described in link:https://docs.github.com/en/webhooks/webhook-events-and-payloads#pull_request[the GitHub docs].
 
 Config orchestration tools are available from within your pipelines are as follows: