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: fc6f89d ("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>

Author: t-8ch

include/linux/stop_machine.h

@@ -99,7 +99,7 @@ static inline void print_stop_info(const char *log_lvl, struct task_struct *task
  * stop_machine: freeze the machine on all CPUs and run this function
  * @fn: the function to run
  * @data: the data ptr to pass to @fn()
- * @cpus: the cpus to run @fn() on (NULL = run on each online CPU)
+ * @cpus: the cpus to run @fn() on (NULL = one unspecified online CPU)
  *
  * Description: This causes a thread to be scheduled on every CPU, which
  * will run with interrupts disabled.  Each CPU specified by @cpus will
@@ -133,7 +133,7 @@ int stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus);
  * stop_machine_cpuslocked: freeze the machine on all CPUs and run this function
  * @fn: the function to run
  * @data: the data ptr to pass to @fn()
- * @cpus: the cpus to run @fn() on (NULL = run on each online CPU)
+ * @cpus: the cpus to run @fn() on (NULL = one unspecified online CPU)
  *
  * Same as above.  Avoids nested calls to cpus_read_lock().
  *