Docs: differences against issueTriggerComment and this plugin

[efip-ns]

Add the differences between this and issueCommentTrigger from "Pipeline: GitHub" plugin.

Testing done

None; this is a change to documentation only.

Submitter checklist

• [?] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed