add comment why tvOS CI job is separate from the matrix (#928) #1002

Job	Run time
Clippy	5s
Test CUDA support	19s
MSRV (ubuntu-latest)	25s
Test aarch64-apple-tvos	2m 20s
MSRV (windows-latest)	42s
Rustfmt	6s
Test (stable)	21s
Test (beta)	26s
Test (nightly)	30s
Test (linux32)	37s
Test (macos)	1m 22s
Test (aarch64-ios)	51s
Test (win32)	1m 11s
Test (win64)	2m 4s
Test (mingw32)	1m 3s
Test (mingw64)	1m 34s
Test (windows-2019)	1m 27s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 7s
	17m 30s

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