Documentation, ABI: Update L3 cache index disable text

Change contact person to AMD kernel mailing list, update text and
external references, drop "Users:" tag.

Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Link: http://lkml.kernel.org/r/1305553188-21061-4-git-send-email-bp@amd64.org
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>