clarifying docs for ToPILImage() #489

Job	Run time
onnx / linux-job	1m 30s
unittests-extended / linux-job	1m 26s
unittests-linux (3.10, linux.12xlarge, cpu) / linux-job	1m 30s
unittests-linux (3.8, linux.12xlarge, cpu) / linux-job	1m 33s
unittests-linux (3.11, linux.12xlarge, cpu) / linux-job	1m 31s
unittests-linux (3.8, linux.g5.4xlarge.nvidia.gpu, cuda, 11.8) / linux-job	3m 27s
unittests-linux (3.9, linux.12xlarge, cpu) / linux-job	1m 34s
unittests-macos (3.10, macos-12) / macos-job	2m 53s
unittests-macos (3.11, macos-12) / macos-job	2m 12s
unittests-macos (3.8, macos-12) / macos-job	2m 42s
unittests-macos (3.8, macos-m1-12) / macos-job	1m 15s
unittests-macos (3.9, macos-12) / macos-job	2m 18s
unittests-windows (3.10, windows.4xlarge, cpu) / windows-job	3m 5s
unittests-windows (3.11, windows.4xlarge, cpu) / windows-job	3m 16s
unittests-windows (3.8, windows.4xlarge, cpu) / windows-job	2m 20s
unittests-windows (3.8, windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job	44s
unittests-windows (3.9, windows.4xlarge, cpu) / windows-job	3m 16s
	36m 32s

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