PullRequest global variable is not shown in Global Variable Reference page.

[Pldi23]

Jenkins and plugins versions report

Environment

Paste the output here

What Operating System are you using (both controller, and any agents involved in the problem)?

I'm using mac, but it's not important.

Reproduction steps

Step 1. Navigate to {JENKINS_URL}/jenkins/job/{pipeline_job_name}/pipeline-syntax/globals and see the list of available global vars, but pullRequest var is missing.

Expected Results

When user navigate to {JENKINS_URL}/jenkins/job/{pipeline_job_name}/pipeline-syntax/globals pullRequest global variable should be listed.

Actual Results

When user navigate to {JENKINS_URL}/jenkins/job/{pipeline_job_name}/pipeline-syntax/globals pullRequest global variable is not listed.

Anything else?

I guess https://github.com/jenkinsci/pipeline-github-plugin/blob/master/src/main/java/org/jenkinsci/plugins/pipeline/github/PullRequestGlobalVariable.java#L15 should be an @Extension and description of all vars should be provided.

I think this improvement will significantly increase the use and recognition of the plugin.