DOC add comment on binsize definition #332

Job	Run time
build (ubuntu-latest, 3.7, gcc, g++)	11m 32s
build (ubuntu-latest, 3.8, gcc, g++)	11m 19s
build (ubuntu-latest, 3.9, gcc, g++)	11m 0s
build (ubuntu-latest, 3.10, gcc, g++)	11m 17s
build (ubuntu-latest, pypy-3.9, gcc, g++)	35m 46s
build (macos-latest, 3.9, cc, c++)	21m 45s
build (ubuntu-latest, 3.8, clang, clang++)	11m 59s
build (ubuntu-latest, 3.9, gcc-11, g++-11)	11m 7s
build (windows-latest, 3.9, gcc, g++)	10m 57s
	2h 16m 42s

Provide feedback