docs: document the opaque struct pattern in one place #1329

Job	Run time
Windows	1m 36s
Windows CMake, Debug configuration	1m 11s
Valgrind	39s
Build+test (clang, stable, ubuntu-20.04)	37s
Clang Tidy	6s
Build+test (clang, beta, ubuntu-20.04)	48s
Clippy	20s
Clippy nightly (optional)	32s
Build+test (clang, nightly, ubuntu-20.04)	44s
Check for documentation errors	17s
Build+test (clang, 1.61.0, ubuntu-20.04)	2m 37s
ensure-header-updated	1m 5s
Build+test (gcc, stable, ubuntu-20.04)	48s
Format	11s
Check minimum versions	38s
Build+test (gcc, beta, ubuntu-20.04)	36s
Build+test (gcc, nightly, ubuntu-20.04)	36s
Miri	54s
Build+test (gcc, 1.61.0, ubuntu-20.04)	2m 36s
Windows CMake, Release configuration	2m 50s
Build+test (macos-latest, clang, stable)	1m 2s
	20m 43s

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