HID: Add HID transport driver documentation

Add David Herrmann's documentation for the new low-level HID transport driver
functions.
Signed-off-by: Frank Praznik <frank.praznik@oh.rr.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>