Add access note to Jira app self-managed doc

What does this MR do and why?

Based on this comment, the MR adds a note to the Jira connect app docs stating that the GitLab instance must be accessible by Jira.

Screenshots or screen recordings

How to set up and validate locally

1. Visit http://localhost:3000/help/integration/jira/connect-app.html#install-the-gitlabcom-for-jira-cloud-app-for-self-managed-instances

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.