dma-buf: Document dynamic locking convention

Add documentation for the dynamic locking convention. The documentation
tells dma-buf API users when they should take the reservation lock and
when not.

Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221017172229.42269-20-dmitry.osipenko@collabora.com