Update the note about the need for COMPAT_FREEBSD<n> kernel options.

Rather than mentioning the requirement for 4.x binaries but not
explaining why (it was assuming an upgrade from 4.x to 5.0-current),
explain when compat options are needed (for running existing host
binaries) in a more general way while using a more modern example
(COMPAT_FREEBSD11 for 11.x binaries). While here, explicitly mention
that a GENERIC kernel should always work.

Reported by: Robert Huff <roberthuff@rcn.com>
Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18740