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