Document getRefPoints and point to it from check.mono in Distance

[LHMarshall]

• If we wanted to give even more information about how these points are selected, I suggest exporting getRefPoints and adding help to it to explian how the ref points are calculated and point to this from check.mono (rather than doing the explanation in check.mono since then if we change getRefPoints we have to remember to change the check.mono documentation.

Originally posted by @lenthomas in #186