signal/alpha: Document a conflict with SI_USER for SIGFPE

signal/alpha: Document a conflict with SI_USER for SIGFPE

Setting si_code to 0 is the same as setting si_code to SI_USER.  This
is the same si_code as SI_USER.  Posix and common sense requires that
SI_USER not be a signal specific si_code.  As such this use of 0 for
the si_code is a pretty horribly broken ABI.

Cc: Helge Deller <deller@gmx.de>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: linux-alpha@vger.kernel.org
History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Ref: 0a635c7a84cf ("Fill in siginfo_t.")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>