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

Pinned, revoked and movable mappings are selected by which optional
callbacks each side implements and by whether dma_buf_pin() succeeds,
not by any flag or enum. Nothing in Documentation/ says so, and the
rules are spread over the kdoc of dma_buf_ops.pin,
dma_buf_attach_ops.invalidate_mappings and dma_buf_invalidate_mappings(),
so a driver author has to know the symbol names before finding them.

Name, per flow, the callbacks both sides have to implement to end up in
it, describe dma_buf_pin() as the runtime negotiation, and record that
the pin is what tells a revoke from a move.

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