docs: explain type of memory allocated by md_per_io_size #939

Job	Run time
build-containers (debian, debian, bookworm, x86_64)	12m 35s
build-containers (fedora, fedora, 40, x86_64)	13m 57s
build (linux-gcc)	14m 11s
build-containers (alma, almalinux, 9, x86_64)	14m 1s
build (linux-clang)	12m 1s
build-containers (oracle, oraclelinux, 9, x86_64)	14m 13s
build (macos)	12m 54s
build-containers (rocky, rockylinux, 9, x86_64)	14m 22s
build (linux-i686-gcc)	13m 6s
build-containers (ubuntu, ubuntu, noble, i686)	13m 14s
build-containers (ubuntu, ubuntu, noble, x86_64)	13m 40s
build (android)	31s
build (windows-cygwin-64)	16m 11s
build (windows-cygwin-32)	15m 56s
build (windows-msys2-64)	16m 30s
build (android-recovery, ubuntu-22.04, aarch64-linux-android32)	31s
	3h 17m 53s

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