bfe: add unmaintained / deprecation notice

bfe: add unmaintained / deprecation notice

The bfe (Broadcom BCM4401 10/100 Ethernet) driver has known bugs and no
active maintenance.  There have been no changes other than sweeping tree
changes, typo corrections etc. since 2008 a far as I can tell.  Add a
note in the man page so that users expectations are correctly set, and
indicate that it may be removed in the future.

I did not add a gone_in() call in the driver itself as there is no
specific target version for removal, and this driver has evidence of
recent use (dmesg, PRs).

PR:		201947, 213751
Reviewed by:	brooks
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40625