Documentation/dma-buf-sharing.txt: update API descriptions

				     size_t size, int flags,

				     const char *exp_name)

   If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a

   pointer to the same. It also associates an anonymous file with this buffer,

   so it can be exported. On failure to allocate the dma_buf object, it returns

   NULL.

   If this succeeds, dma_buf_export_named allocates a dma_buf structure, and

   returns a pointer to the same. It also associates an anonymous file with this

   buffer, so it can be exported. On failure to allocate the dma_buf object,

   it returns NULL.

   'exp_name' is the name of exporter - to facilitate information while

   debugging.

   drivers and/or processes.

   Interface:

      int dma_buf_fd(struct dma_buf *dmabuf)

      int dma_buf_fd(struct dma_buf *dmabuf, int flags)

   This API installs an fd for the anonymous file associated with this buffer;

   returns either 'fd', or error.

   "dma_buf->ops->" indirection from the users of this interface.

   In struct dma_buf_ops, unmap_dma_buf is defined as

      void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *);

      void (*unmap_dma_buf)(struct dma_buf_attachment *,

                            struct sg_table *,

                            enum dma_data_direction);

   unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like

   map_dma_buf, this API also must be implemented by the exporter.