fdisk: add deprecation/removal notice to man page
As of 2015 (commit b8c19fd71994) the fdisk man page claimed the command
is obsolete, but had no explicit mention of removal. Add another note
that follows our current deprecation notice format.
Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47067
(cherry picked from commit 255d2d9bbc3756c940462c61d3cd6fb7753aab2c)