doc: document handling of dynamically sized types #1290

Job	Run time
Valgrind	1m 24s
Build+test (clang, stable, ubuntu-20.04)	41s
Build+test (clang, beta, ubuntu-20.04)	38s
Windows	1m 45s
Build+test (clang, nightly, ubuntu-20.04)	37s
Windows CMake, Debug configuration	1m 57s
Windows CMake, Release configuration	2m 15s
Build+test (clang, 1.61.0, ubuntu-20.04)	4m 13s
Check for documentation errors	17s
Build+test (gcc, stable, ubuntu-20.04)	36s
ensure-header-updated	1m 3s
Check minimum versions	34s
Build+test (gcc, beta, ubuntu-20.04)	34s
Build+test (gcc, nightly, ubuntu-20.04)	36s
Build+test (gcc, 1.61.0, ubuntu-20.04)	4m 52s
Build+test (macos-latest, clang, stable)	1m 12s
Clang Tidy	6s
Clippy	20s
Clippy nightly (optional)	16s
Format	13s
Miri	49s
	24m 58s

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