Clarify reduction documentation (#82)

* clarify reduction documentation

* elaborate on associativity of reduce_func

* add associativity example

* remove docstring duplication

Co-authored-by: Alex Fikl <alexfikl@gmail.com>

Co-authored-by: Andreas Klöckner <inform@tiker.net>
Co-authored-by: Alex Fikl <alexfikl@gmail.com>