fs/resctrl: Document tasks file behaviour for task id 0 and idle tasks

fs/resctrl: Document tasks file behaviour for task id 0 and idle tasks

When 0 is written to the tasks file it is interpreted as the current task in
rdtgroup_move_task(). Each CPU's idle task has task_struct::pid set to 0 and,
on x86, task_struct::closid to RESCTRL_RESERVED_CLOSID and task_struct::rmid
to RESCTRL_RESERVED_RMID.

Equivalently, on MPAM platforms, thread_info::mpam_partid_pmg is encoded with
PARTID and PMG set to RESCTRL_RESERVED_CLOSID and RESCTRL_RESERVED_RMID,
respectively. As there is no interface to change these from the default, the
resctrl configuration for the idle tasks is fixed and they always behave
equivalently to a task in the default tasks file and so take their
configuration from the cpus/cpus_list files.

On read of the tasks file, show_rdt_tasks() filters out any 0 PID. Hence, a
task id of 0 is never shown in the tasks file and the idle tasks are not
represented either.

Document the user visible behaviour.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Babu Moger <babu.moger@amd.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Link: https://lore.kernel.org/20260506082855.3694761-1-ben.horgan@arm.com