dma-buf/dma_resv_usage: update explicit sync documentation

Make it clear that DMA_RESV_USAGE_BOOKKEEP can be used for explicit synced
user space submissions as well and document the rules around adding the
same fence with different usages.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220712131201.131475-1-christian.koenig@amd.com