docs: usbip: Fix major fields and descriptions in protocol

The old document for usbip protocol is misleading and hard to read:
  * Some fields in header are incorrect
  * Explanation of some fields are unclear or even wrong
  * Padding of header (namely all headers have the same length) is
    not explicitly pointed out, which is crucial for stream protocol
    like TCP

Major changes:
  * Document the correct field as described in the codebase.
  * Document the padding in usbip headers. This is crucial for TCP
    stream hence these padding should be explicitly point out.
    In code these padding are implemented by a union of all headers.
  * Fix two FIXME related to usbip unlink and Document the behavior
    of unlink in different situation.
  * Clarify some field with more accurate explanation, like those
    fields associated with URB. Some constraints are extracted from
    code.
  * Delete specific transfer_flag doc in usbip as it should be
    documented by the URB doc in Documentation/driver-api/...