More clarity on niche numerics

https://javabook.mccue.dev/niche_numerics/short
could add a footnote why short is called short (and long long) and why the range is these weird arbitrary numbers. (i.e. just a note 16 bits, 32 bits, 64 bits without going into detail what that means, i guess)

originally suggested in the TJ discord by zabuzard.