docs: driver-api: usb: avoid using ReST :doc:`foo` markup

The big picture is that USB drivers can continue to ignore most DMA issues,

though they still must provide DMA-ready buffers (see

:doc:`/core-api/dma-api-howto`).  That's how they've worked through

Documentation/core-api/dma-api-howto.rst).  That's how they've worked through

the 2.4 (and earlier) kernels, or they can now be DMA-aware.

DMA-aware usb drivers:

  force a consistent memory access ordering by using memory barriers.  It's

  not using a streaming DMA mapping, so it's good for small transfers on

  systems where the I/O would otherwise thrash an IOMMU mapping.  (See

  :doc:`/core-api/dma-api-howto` for definitions of "coherent" and

  Documentation/core-api/dma-api-howto.rst for definitions of "coherent" and

  "streaming" DMA mappings.)

  Asking for 1/Nth of a page (as well as asking for N pages) is reasonably

Existing buffers aren't usable for DMA without first being mapped into the

DMA address space of the device.  However, most buffers passed to your

driver can safely be used with such DMA mapping.  (See the first section

of :doc:`/core-api/dma-api-howto`, titled "What memory is DMA-able?")

of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?")

- When you're using scatterlists, you can map everything at once.  On some

  systems, this kicks in an IOMMU and turns the scatterlists into single