docs: clarify cuda async execution model #6611

Performance Gate Passed

⚡ 1 improved benchmark
✅ 985 untouched benchmarks
⏩ 1426 skipped benchmarks¹

	Mode	Benchmark	`BASE`	`HEAD`	Efficiency
⚡	Simulation	`map_each[BufferMut<i32>, 128]`	887.2 ns	799.7 ns	+10.94%

_{Comparing ad/cuda-async-docs (64e9d97) with develop (ab55ce4)}

1426 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩