Documentation: usb: Document FunctionFS DMABUF API

Add documentation for the three ioctls used to attach or detach
externally-created DMABUFs, and to request transfers from/to previously
attached DMABUFs.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20240130122340.54813-5-paul@crapouillou.net


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>