Documentation: sysfs-bus-usb: update power/persist description

There's no power/persist file for hubs. And CONFIG_USB_PERSIST was
removed in v2.6.26. Update the description of power/persist accordingly.
Also remove the line on its default value. It is not entirely correct, as
CONFIG_USB_DEFAULT_PERSIST and the USB_QUIRK_RESET flag influence the
default. It is not needed to understand this file anyhow.
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/stable/sysfs-bus-usb

@@ -3,13 +3,13 @@ Date:		May 2007
 KernelVersion:	2.6.23
 Contact:	Alan Stern <stern@rowland.harvard.edu>
 Description:
-		If CONFIG_USB_PERSIST is set, then each USB device directory
-		will contain a file named power/persist.  The file holds a
-		boolean value (0 or 1) indicating whether or not the
-		"USB-Persist" facility is enabled for the device.  Since the
-		facility is inherently dangerous, it is disabled by default
-		for all devices except hubs.  For more information, see
-		Documentation/usb/persist.txt.
+		USB device directories can contain a file named power/persist.
+		The file holds a boolean value (0 or 1) indicating whether or
+		not the "USB-Persist" facility is enabled for the device.  For
+		hubs this facility is always enabled and their device
+		directories will not contain this file.
+
+		For more information, see Documentation/usb/persist.txt.
 
 What:		/sys/bus/usb/devices/.../power/autosuspend
 Date:		March 2007