stop_machine: Fix the documentation for a NULL cpus argument

A recent refactoring of the kernel-docs for stop machine changed the
description of the cpus parameter from "NULL = any online cpu"
to "NULL = run on each online CPU".

However the callback is only executed on a single CPU, not all of them.
The old wording was a bit ambiguous and could have been read both ways.

Reword the documentation to be correct again and hopefully also clearer.

Fixes: fc6f89dc7078 ("stop_machine: Improve kernel-doc function-header comments")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>