prop#324: mention rearranged form for N_EWMA smoothing
The formula used for N_EWMA is actually rearranged in the C tor implementation, in a way that caused the arti reimplementation to be off by a small rounding error until it was corrected.
In order to ensure other implementations don't have this issue, mention the rearranged form and add a requirement that it be used.
(background: arti!525 (comment 2807244))