docs: stable-ABI: Document /sys/kernel/notes

Document the notes file in sysfs as the running vmlinux's .note section
in binary format. Hopefully this helps someone like me realize the
kernel exposes the note section in sysfs in the future. Take the date
from when the file was introduced. It's been a while so presumably this
is stable and not testing material.

Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20200909063752.931283-1-swboyd@chromium.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>