PM / Runtime: Improve documentation of enable, disable and barrier

The runtime PM documentation in Documentation/power/runtime_pm.txt
doesn't say that pm_runtime_enable() and pm_runtime_disable() work by
operating on power.disable_depth, which is wrong, because the
possibility of nesting disables doesn't follow from the description
of these functions.  Also, there is no description of
pm_runtime_barrier() at all in the document, which is confusing.
Improve the documentation by fixing those issues.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

Documentation/power/runtime_pm.txt

@@ -369,17 +369,27 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h:
       pm_runtime_autosuspend(dev) and return its result
 
   void pm_runtime_enable(struct device *dev);
-    - enable the run-time PM helper functions to run the device bus type's
-      run-time PM callbacks described in Section 2
+    - decrement the device's 'power.disable_depth' field; if that field is equal
+      to zero, the run-time PM helper functions can execute subsystem-level
+      callbacks described in Section 2 for the device
 
   int pm_runtime_disable(struct device *dev);
-    - prevent the run-time PM helper functions from running subsystem-level
-      run-time PM callbacks for the device, make sure that all of the pending
+    - increment the device's 'power.disable_depth' field (if the value of that
+      field was previously zero, this prevents subsystem-level runtime PM
+      callbacks from being run for the device), make sure that all of the pending
       run-time PM operations on the device are either completed or canceled;
       returns 1 if there was a resume request pending and it was necessary to
       execute the subsystem-level resume callback for the device to satisfy that
       request, otherwise 0 is returned
 
+  int pm_runtime_barrier(struct device *dev);
+    - check if there's a resume request pending for the device and resume it
+      (synchronously) in that case, cancel any other pending runtime PM requests
+      regarding it and wait for all runtime PM operations on it in progress to
+      complete; returns 1 if there was a resume request pending and it was
+      necessary to execute the subsystem-level resume callback for the device to
+      satisfy that request, otherwise 0 is returned
+
   void pm_suspend_ignore_children(struct device *dev, bool enable);
     - set/unset the power.ignore_children flag of the device