Collapse three contiguous comment blocks into one. Remove historical
note about wrong assumptions 20 years ago. Use proper casing.

Sponsored by:	Nginx, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274652 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Re-work non-persistent filesystem detection as it was not possible to
detect /dev/md backed mfs filesystems that way.

Differential Revision:	https://reviews.freebsd.org/D1163
Approved by:	ian


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274653 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f