Update install documentation to include FreeBSD
Summary:
Fixes T4024.
- Mention FreeBSD by name.
- Make "install this on a normal computer" more explicit.
- Make "install this on a entire domain" more explicit.
Test Plan:
{F74892}
{F74891}
Reviewers: btrahan, chad
Reviewed By: chad
CC: aran
Maniphest Tasks: T4024
Differential Revision: https://secure.phabricator.com/D7397