R1:5c8dabdf8034

Add a strong hint about importing or observing repositories to repository creation

Summary: Fixes T11278. Also mention `svnsync`, since we have some evidence that it works.

Test Plan: {F1716250}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11278

Differential Revision: https://secure.phabricator.com/D16255