Document what `Build::target` does #54

Job	Run time
Test CUDA support	1m 19s
Test build-std (aarch64-tvos)	2m 5s
Test wasm (wasm32-unknown-unknown)	19s
Test build-std (aarch64-tvos-sim)	2m 24s
Clippy	11s
Test build-std (x86_64-tvos)	1m 57s
MSRV (ubuntu-latest)	2m 17s
Rustfmt	8s
MSRV (windows-latest)	2m 56s
semver-checks	2m 40s
MSRV (macos-latest)	2m 23s
Test (stable)	1m 56s
Test wasm32-wasip1-thread	32s
Test (beta)	2m 2s
Test (nightly)	1m 59s
Test (linux32)	2m 12s
Test (macos)	3m 17s
Test (aarch64-macos)	1m 21s
Test (aarch64-ios)	1m 39s
Test (win32)	3m 39s
Test (win64)	3m 11s
Test (mingw32)	3m 48s
Test (mingw64)	3m 57s
Test (windows-2019)	4m 34s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	3m 51s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	6m 21s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	4m 28s
	1h 7m 26s

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