cgroup/rstat: desc member cgrp in cgroup_rstat_flush_release

Recent change to cgroup_rstat_flush_release added a
parameter cgrp, which is used by tracepoint to correlate
with other tracepoints that also have this cgrp.

The kernel test robot detected kernel doc was missing
a description of this member.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404170821.HwZGISTY-lkp@intel.com/
Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>