agp.4: Add a deprecation notice to the man page

The driver is giant-locked and thus already prints a deprecation warning
when attaching. The device file interface was broken in 14.0 and 14.1,
see commit 12500c14281d, but it took a very long time for anyone to
notice, and in that case it was only because of some code which probes
all device files.

Relnotes: yes
Reviewed by: imp, jhb, emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46542