Merge pull request ARMmbed#11635 from hugueskamba/hk-fix-deepsleep-us…

…b-components

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

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();