fmtree: add a deprecation notice to the manpage

Note that this mtree(8) is actually installed as fmtree(8), while
mtree(8) is located in ^/contrib/mtree -- thus, the reference to
mtree(8) makes a lot more sense in the context in which folks would
actually notice it. Shout-out to Ravi for pointing out that this may
not be an obvious fact.

MFC after: 3 days
Reviewed by: bdrewery, brooks, cy, emaste
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D28573