docs on the curve plot

[nick-youngblut]

It would be great if https://nonpareil.readthedocs.io/en/latest/curves.html#nonpareil-curves included a detailed description (or link to such a description) of the curve plot generated by Nonpareil.set() (e.g., the arrows, red-dashed horizontal lines, etc). For example: "see the Figure S1 caption in the Nonpareil3 paper"