comment: threat .j2 files as Jinja2

[Thannoy]

Ansible commonly uses .j2 file extensions for Jinja2 template files it relies on.

Link: https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_templating.html

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• [x] Added a change log entry in changelog.d/<directory>/.
• [ ] Added self to copyright blurb of touched files. addition pending in another MR, not added here to avoid conflicts
• [x] Added self to AUTHORS.rst.
• [ ] Wrote tests.
• [ ] Documented my changes in docs/man/ or elsewhere.
• [x] My changes do not contradict the current specification.
• [x] I agree to license my contribution under the licenses indicated in the changed files.