📚 Document `counting_iterator` #962

Job	Run time
mutate	5m 15s
valgrind	5m 41s
sanitize (address, gcc)	4m 49s
sanitize (undefined, clang)	5m 41s
quality_checks_pass	53s
sanitize (address, clang)	5m 44s
sanitize (thread, gcc)	4m 47s
build_and_test_24 (clang, 19, 23, libstdc++, Debug)	5m 41s
build_and_test_24 (clang, 21, 23, libstdc++, Debug)	5m 31s
build_and_test_24 (clang, 20, 23, libc++, Debug)	6m 23s
sanitize (undefined, gcc)	4m 52s
build_and_test_24 (gcc, 12, 23, libstdc++, Debug)	4m 28s
build_and_test_24 (clang, 18, 23, libstdc++, Debug)	5m 48s
sanitize (thread, clang)	6m 6s
build_and_test_24 (gcc, 14, 23, libstdc++, Debug)	5m 19s
build_and_test_24 (clang, 21, 23, libc++, Debug)	5m 55s
build_and_test_24 (clang, 18, 23, libc++, Debug)	7m 23s
build_and_test_24 (clang, 22, 23, libstdc++, Debug)	6m 14s
build_and_test_24 (clang, 22, 23, libc++, Debug)	6m 15s
build_and_test_24 (gcc, 13, 23, libstdc++, Debug)	4m 47s
build_and_test_24 (clang, 20, 23, libstdc++, Debug)	6m 1s
build_and_test_24 (clang, 19, 23, libc++, Debug)	6m 31s
merge_ok	4s
	2h 0m 8s

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