Documentation: Add ABI entry for crash_notes and crash_notes_size

Add an Documentation/ABI entry for /sys/devices/system/cpu/cpu0/crash_notes
and /sys/devices/system/cpu/cpu0/crash_notes_size.

Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>