docs: Document I idle state in ps and top

Linux 4.2 provided a new process state of I which is used for an idle
kernel thread. This new state means that kernel threads do not
contribute to the loadavg as they are no longer state D or S but I.

While both ps and top displayed this state, it wasn't documented in
either manual page until now.

References:
 https://bugs.debian.org/886967
 https://www.quora.com/What-does-mean-Linux-process-state-I-in-the-top-output
 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=80ed87c8a9ca0cad7ca66cf3bbdfb17559a66dcf