dma-buf: doc polish for pin/unpin

Motivated by a discussion with Christian and Thomas: Try to untangle a
bit what's relevant for importers and what's relevant for exporters.

Also add an assert that really only dynamic importers use the api
function, anything else doesn't make sense.

Acked-by: Christian König <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: Sumit Semwal <[email protected]>
Cc: "Christian König" <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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