docs: document the opaque struct pattern in one place #1336

Job	Run time
Clippy nightly (optional)	18s
Clippy	17s
Build+test (clang, stable, ubuntu-20.04)	48s
Build+test (clang, beta, ubuntu-20.04)	42s
Clang Tidy	6s
Check for documentation errors	16s
Build+test (clang, nightly, ubuntu-20.04)	43s
ensure-header-updated	1m 2s
Build+test (clang, 1.61.0, ubuntu-20.04)	1m 15s
Format	13s
Build+test (gcc, stable, ubuntu-20.04)	38s
Check minimum versions	37s
Miri	1m 2s
Build+test (gcc, beta, ubuntu-20.04)	36s
Build+test (gcc, nightly, ubuntu-20.04)	41s
Windows	1m 33s
Windows CMake, Debug configuration	1m 9s
Windows CMake, Release configuration	1m 47s
Valgrind	50s
Build+test (gcc, 1.61.0, ubuntu-20.04)	1m 7s
Build+test (macos-latest, clang, stable)	1m 29s
	17m 9s

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