Correct the truncate documentation on 4D types #1005

Job	Run time
Lint	3m 54s
Test portable simd (ubuntu-latest, nightly)	1m 17s
Test portable simd (macos-latest, nightly)	4m 11s
Test portable simd (windows-latest, nightly)	2m 30s
Test (ubuntu-latest, stable)	3m 17s
Test (ubuntu-latest, beta)	3m 57s
Test (ubuntu-latest, nightly)	3m 15s
Test (macos-latest, stable)	6m 46s
Test (macos-latest, beta)	7m 9s
Test (macos-latest, nightly)	8m 50s
Test (windows-latest, stable)	6m 22s
Test (windows-latest, beta)	7m 33s
Test (windows-latest, nightly)	6m 18s
Test wasm (stable, ubuntu-latest)	5m 6s
	1h 10m 25s

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