CPU/hotplug: Clarify description of __cpuhp_setup_state() return value

When ivoked with CPUHP_AP_ONLINE_DYN state __cpuhp_setup_state()
is expected to return positive value which is the hotplug state that
the routine assigns.
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/cpu.c

@@ -1586,7 +1586,11 @@ EXPORT_SYMBOL_GPL(__cpuhp_state_add_instance);
  * @startup:	startup callback function
  * @teardown:	teardown callback function
  *
- * Returns 0 if successful, otherwise a proper error code
+ * Returns:
+ *   On success:
+ *      Positive state number if @state is CPUHP_AP_ONLINE_DYN
+ *      0 for all other states
+ *   On failure: proper (negative) error code
  */
 int __cpuhp_setup_state(enum cpuhp_state state,
 			const char *name, bool invoke,