message: Update PR template with link to docs
diff: * [ ] Closes #xxx
  * [ ] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
  * [ ] Tests added/passed
  -* [ ] Requires documentation to be updated
  +* [ ] Requires documentation to be updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
  * [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx
  <!-- Provide a more detailed description of the PR, other things fixed or any additional comments/features here -->
  