Add doc comment to `Quat::is_nan` #1441

Job	Run time
Lint	2m 9s
Check MSRV (ubuntu-latest, 1.68.2)	1m 1s
Test portable simd (ubuntu-latest, nightly)	43s
Check MSRV (macos-latest, 1.68.2)	1m 0s
Test (ubuntu-latest, stable)	3m 2s
Test portable simd (macos-latest, nightly)	41s
Check MSRV (windows-latest, 1.68.2)	1m 26s
Test (ubuntu-latest, beta)	2m 55s
Test wasm (stable, ubuntu-latest)	4m 51s
Test portable simd (windows-latest, nightly)	1m 31s
Test (ubuntu-latest, nightly)	2m 40s
Test (macos-latest, stable)	2m 58s
Test (macos-latest, beta)	2m 25s
Test (macos-latest, nightly)	2m 27s
Test (windows-latest, stable)	5m 48s
Test (windows-latest, beta)	4m 40s
Test (windows-latest, nightly)	4m 42s
	44m 59s

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