When reviewing Pull Requests (PR) on Github, the common options are to (1) comment (either in-line or top level comments), (2) approve, and (3) request changes. The triggers offered by Jira to automate actions based off of PRs are the following:
PR Created
PR Declined
PR Merged
I haven't yet looked into whether finishing a PR review with Comment or Request changes will trigger the PR Declined hook, but I believe it may be best for those to be separate, as Github has them separate.