sched/core: Add kernel-doc for set_cpus_allowed_ptr()

This is an exported symbol, so it should have kernel-doc.
Add a note to very similar function do_set_cpus_allowed()
to avoid confusion and misuse.

Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230829082551.2661290-1-costa.shul@redhat.com