Provide some more context hints for repository URIs

Summary: Ref T10923. This provides a little guidance about hosted vs observed, and points at the diffusion.ssh-* options.

Test Plan: Poked around in the web UI, saw useful guidance.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10923

Differential Revision: https://secure.phabricator.com/D15872