Document helper macros #1295

Job	Run time
Valgrind	43s
Build+test (clang, stable, ubuntu-20.04)	43s
ensure-header-updated	1m 3s
Check for documentation errors	16s
Build+test (clang, beta, ubuntu-20.04)	39s
Windows	1m 34s
Build+test (clang, nightly, ubuntu-20.04)	43s
Build+test (clang, 1.61.0, ubuntu-20.04)	4m 27s
Windows CMake, Debug configuration	1m 7s
Windows CMake, Release configuration	3m 5s
Build+test (gcc, stable, ubuntu-20.04)	40s
Build+test (gcc, beta, ubuntu-20.04)	35s
Clang Tidy	9s
Build+test (gcc, nightly, ubuntu-20.04)	33s
Clippy	16s
Build+test (gcc, 1.61.0, ubuntu-20.04)	4m 8s
Clippy nightly (optional)	19s
Build+test (macos-latest, clang, stable)	1m 47s
Format	16s
Check minimum versions	39s
Miri	51s
	24m 33s

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