reactor, linux-aio: advise users in more detail on setting `aio-max-nr` #455

Job	Run time
Test with C++20 modules enabled / test	12m 36s
Test with DPDK enabled / test	18m 15s
Test (clang++, C++20, debug) / test	12m 23s
Test (clang++, C++20, dev) / test	10m 53s
Test (clang++, C++20, release) / test	11m 24s
Test (clang++, C++23, dev) / test	11m 14s
Test (clang++, C++23, debug) / test	12m 17s
Test (clang++, C++23, release) / test	11m 22s
Test (g++, C++20, dev) / test	9m 20s
Test (g++, C++23, debug) / test	14m 37s
Test (g++, C++20, debug) / test	15m 2s
Test (g++, C++20, release) / test	12m 39s
Test (g++, C++23, dev) / test	9m 33s
Test (g++, C++23, release) / test	12m 52s
	2h 54m 27s

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