Rewrite some of the disk setup documentation to be clearer and contain
less obsolete information. Also move the entropy command down the list --
the list is ordered by likelihood of use rather than alphabetically.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273922 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f