doc: dma-buf: Rewrite intro section a little

Make it a little bit more clear what's going on and fix some formatting.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20230803154908.105124-3-daniels@collabora.com