PM: sleep: Add kerneldoc comments to some functions

Add kerneldoc comments to pm_suspend_via_firmware(),
pm_resume_via_firmware() and pm_suspend_via_s2idle()
to explain what they do.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/power/suspend.c

@@ -62,6 +62,12 @@ static DECLARE_SWAIT_QUEUE_HEAD(s2idle_wait_head);
 enum s2idle_states __read_mostly s2idle_state;
 static DEFINE_RAW_SPINLOCK(s2idle_lock);
 
+/**
+ * pm_suspend_via_s2idle - Check if suspend-to-idle is the default suspend.
+ *
+ * Return 'true' if suspend-to-idle has been selected as the default system
+ * suspend method.
+ */
 bool pm_suspend_via_s2idle(void)
 {
 	return mem_sleep_current == PM_SUSPEND_TO_IDLE;