USBDevice: add documentation on USB suspend/resume to enter deep sleep

It is possible to temporarily suspend USB and safely preserve its
configuration. This is needed to allow a device to enter deep
sleep as a USBDevice instance prevents deep sleep. USB operation can be
suspended with `deinit` and restored with `connect`.

Author: hugueskamba

drivers/internal/USBDevice.h

@@ -97,6 +97,11 @@ class USBDevice: public  USBPhyEvents {
      * Power down this instance
      *
      * Disable interrupts and stop sending events.
+     * This method can be used for temporary power-saving; This call can allow
+     * USB to be temporarily disabled to permit power saving.
+     * However, it is up to the user to make sure all the
+     * transfers have concluded (for example when USB power is lost).
+     * USBDevice::connect can be used to resume USB operation.
      */
     void deinit();
 
@@ -109,6 +114,8 @@ class USBDevice: public  USBPhyEvents {
 
     /**
     * Connect a device
+    * This method can also be used to resume USB operation when USB power is
+    * detected after it was suspended via USBDevice::deinit.
     */
     void connect();