Document helper macros #1301

Job	Run time
Build+test (clang, stable, ubuntu-20.04)	47s
Clang Tidy	8s
Clippy	21s
Build+test (clang, beta, ubuntu-20.04)	44s
Build+test (clang, nightly, ubuntu-20.04)	53s
Clippy nightly (optional)	22s
Check for documentation errors	16s
Build+test (clang, 1.61.0, ubuntu-20.04)	4m 21s
ensure-header-updated	1m 9s
Build+test (gcc, stable, ubuntu-20.04)	41s
Build+test (gcc, beta, ubuntu-20.04)	37s
Format	14s
Check minimum versions	37s
Build+test (gcc, nightly, ubuntu-20.04)	36s
Miri	50s
Build+test (gcc, 1.61.0, ubuntu-20.04)	4m 18s
Windows	3m 29s
Windows CMake, Debug configuration	2m 27s
Build+test (macos-latest, clang, stable)	59s
Windows CMake, Release configuration	2m 27s
Valgrind	55s
	27m 11s

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