Jira DVCS docs - Call out differences between 11.3 and before

What does this MR do?

Call out differences between the configuration for Jira DVCS between 11.3 and earlier.

Also adds a small reminder to ensure self-hosted GitLab is accessible from Jira.

What are the relevant issue numbers?

#7400 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #7400 (closed)