cpumask: add a few comments of cpumask functions

Current few cpumask functions' purposes are not quite clear. Stupid
user like myself needs to dig into details for clear function
purpose and return value.
Add few explanation for them is helpful.

Thanks for Srivatsa's comments and correction!

Signed-off-by: Alex Shi <alex.shi@intel.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>