Clarify the behavior of UsbRequest

This is obviously just about describing a bad behavior. In the next step
these methods should be deprecated and replaced with versions that deal
with ByteBuffers appropriately.

Bug: 31050148
Test: Test that enshrines the documented behavior is submitted alongside
      this change
Change-Id: If250a8bbd636784355e839a1638d52f3bbe9b83d