Merge #10577: Add an explanation of quickly hashing onto a non-power of two range.

Summary:
dd869c6 Add an explanation of quickly hashing onto a non-power of two range. (Gregory Maxwell)

Tree-SHA512: 8b362e396206a4ee2e825908dcff6fe4525c12b9c85a6e6ed809d75f03d42edcfba5e460a002e5d17cc70c103792f84d99693563b638057e4e97946dd1d800b2

Backport of Core PR10577
https://github.com/bitcoin/bitcoin/pull/10577

Test Plan:

make check

Reviewers: jasonbcox, deadalnix, Fabien, O1 Bitcoin ABC, #bitcoin_abc, markblundeberg

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3356