Add clarification points to using SCP style URIs

What does this MR do?

Add clarification points to using SCP style URIs

Are there points in the code the reviewer needs to double check?

No. This updates a doc for repository mirroring and a validation hint in the UI.

Why was this MR needed?

This addresses user-facing documentation related to repository mirroring.

On the repositoring mirroring view of repository settings, inputting either git@example.com:group/project.git or any variation of schemes:

• https://git@example.com:group/project.git
• http://git@example.com:group/project.git
• git://git@example.com:group/project.git
• ssh://git@example.com:group/project.git

Fails on URL validation.

This commit adds notes about the required format to the doc and view for repository mirroring. This commit does not attempt to validate the URL, so does not resolve the linked issue.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#47544 (closed)