dma-buf: Document how exporters and importers agree on mapping lifetime

Nothing in DMA-buf declares whether a mapping may be moved or taken away.
Nearly every callback on both sides is optional, so the answer follows
from which ones are implemented and from whether dma_buf_pin() succeeds.
Documentation/ says none of this, and the rules sit in the kdoc of
dma_buf_ops.pin, dma_buf_attach_ops.invalidate_mappings and
dma_buf_invalidate_mappings(), which a driver author has to know by name
before finding them.

Describe the three flows an importer has to handle, the callbacks each
one asks of both sides, and dma_buf_pin() as the runtime negotiation.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>