dma-buf: Document struct dma_buf_map

This patch adds struct dma_buf_map and its helpers to the documentation. A
short tutorial is included.

v3:
	* update documentation in a separate patch
	* expand docs (Daniel)
	* carry-over acks from patch 1

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200925115601.23955-5-tzimmermann@suse.de