Update vhost.md

[idfubar]

Revised 'Groups, faculty, and academic staff only' to include a parenthetical (reading "see [Google Domains Help][domain-forwarding] for an example"); also revised 'Limitations on non-berkeley.edu' to include a more descriptive sub-heading ('Limitations on non-berkeley.edu domains'), to include an example ('www.foobarbaz.com'), and to read as documentation of a reserved capacity exercised in exceptional circumstances (rather than, say, a framework by which regular requests for exception should be expected)