[llvm] [NFC][Offload] Clarify `olDestroyQueue` (PR #152132)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

@callumfare Looks like HSA is the opposite, it's UB to delete a queue while there is still work happening. All the better to formalize it in liboffload, I guess.

If the user wants to block until the queue is destroyed, then they can always do `olSyncronize` before destroying the queue, but there is no easy way for them to throw away a queue once work is done.

However, now that I'm looking at a hypothetical olEnqueueCallback, loading up a callback may have a lot of overhead compared to just syncing... So I'm not sure what the play is here.

https://github.com/llvm/llvm-project/pull/152132