R1:623ed1f4349b

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