Update jira_connect_oauth ff documentation

What does this MR do and why?

Related to: #355537 (closed)

This issue changes the documentation of the jira_connect_oauth feature flag to discourage using it in a production environment.

Screenshots or screen recordings

How to set up and validate locally

1. Visit http://localhost:3000/help/development/integrations/jira_connect

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.