doc: document handling of dynamically sized types #1294

Job	Run time
Windows CMake, Debug configuration	1m 50s
Build+test (clang, stable, ubuntu-20.04)	37s
Build+test (clang, beta, ubuntu-20.04)	40s
ensure-header-updated	1m 1s
Build+test (clang, nightly, ubuntu-20.04)	39s
Check for documentation errors	16s
Check minimum versions	34s
Build+test (clang, 1.61.0, ubuntu-20.04)	4m 14s
Windows	1m 32s
Build+test (gcc, stable, ubuntu-20.04)	35s
Windows CMake, Release configuration	2m 25s
Build+test (gcc, beta, ubuntu-20.04)	34s
Valgrind	43s
Build+test (gcc, nightly, ubuntu-20.04)	34s
Clang Tidy	6s
Clippy	17s
Build+test (gcc, 1.61.0, ubuntu-20.04)	4m 10s
Clippy nightly (optional)	17s
Format	21s
Build+test (macos-latest, clang, stable)	1m 13s
Miri	51s
	23m 29s

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