sysfs: add documentation to cputopology.txt for system cpumasks

Add information to cputopology.txt explaining the output of various
system cpumask's.

Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>