Make the field of `ParamSetBuilder` pub so it's actually usable.

[chescock]

Objective

ParamSetBuilder is supposed to be used as a tuple constructor, but the field was not marked pub so it's not actually usable outside of its module.

Solution

Mark the field pub.

Realize one advantage of doc tests over unit tests is that they test the public API.

Add a doc test example that uses the field so that this would have been caught.