anothe small documentation fix

The "using a generator function" part does not make sense to the user after it is not anymore exposed that a Generator is a function of type so-and-so. One could say something like "Create a list of random values using a provided generator for list elements." But the documentation for pair also just says "Create a pair of random values.", not "... using two provided generators for the components".
Merge pull request #37 from jvoigtlaender/patch-2

another small documentation fix