How to use comments to trigger rebuilds of Bitbucket pull-requests on Jenkins multi-branch pipeline
I personally use the Pull Request Notifier plugin for Bitbucket to trigger my Bitbucket Multibranch Pipeline builds in Jenkins.  Using the GUI configuration tools, you can configure the Pull Request Notifier plugin to trigger a build on any arbitrary PR action, including adding comments.

However, you don't really need to use comments to trigger builds - there is a build trigger button in the PR view in Bitbucket, and you can also use the "Build Now" button in Jenkins to trigger a PR build.  Both of these seem a little bit easier to me than a magic comment.