dma-buf: clarify dma_fence_ops->wait documentation

This callback is pretty much deprecated and should not be used by new implementations.

Clarify that in the documentation as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210901120240.7339-2-christian.koenig@amd.com