Document mset.Size versus len(mset) in Python (and Julia?)

[dannys4]

This is something that isn't very well documented and has tripped me up before, and has now tripped up @rubiop on an example. This should probably be documented well because using the wrong one can cause insidious and misleading behavior.

Honestly, we may want to think about renaming, since "size" could easily mean dimension of input or number of elements in the set and "length" honestly seems to me like "number of elements in the set". If we did this, it would (clearly) be breaking so renaming is a change that we save for 3.0. Personally, I would suggest using Cardinality instead of Size to indicate that it's the number of elements in a set, and Dim(ension) instead of Length to show it's about dimension, but those are my two cents.