Include directory-ownership note in sshd setup instructions

Summary: Fixes T9560. We suggest a root-owned location, but users who choose their own location instead can run into trouble.

Test Plan:

• Changed parent directory to have an non-root owner, verified that ssh no longer worked.
• Changed parent directory back to a root owner, verified ssh worked again.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9560

Differential Revision: https://secure.phabricator.com/D15794