`CC_SHELL_ESCAPED_FLAGS`: improve docs #1801

Job	Run time
Test (stable)	44s
Test (beta)	48s
Clippy	11s
Test (nightly)	1m 23s
Test wasm (wasm32-unknown-unknown)	23s
Test build-std (aarch64-tvos)	1m 38s
Test (linux32)	1m 23s
Test build-std (aarch64-tvos-sim)	1m 43s
Test CUDA support	42s
Test (macos)	2m 37s
Test build-std (x86_64-tvos)	2m 23s
Rustfmt	9s
Test (aarch64-macos)	1m 10s
Test (aarch64-ios)	45s
semver-checks	2m 46s
Test (win32)	2m 12s
MSRV (ubuntu-latest)	28s
Test (win64)	2m 40s
MSRV (windows-latest)	41s
Test (mingw32)	2m 14s
MSRV (macos-latest)	32s
Test (mingw64)	1m 36s
Test (windows-2019)	3m 37s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	1m 59s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	3m 6s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	1m 54s
	39m 44s

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