[pyos][docs] "Monoids are also associative, so the order of the inputs does not matter."

[Here](https://python-graphblas.readthedocs.io/en/stable/user_guide/operators.html#monoids) it says,

> Monoids are also associative, so the order of the inputs does not matter.

This is a bit confusing. Did you mean to say that the monoids in py-graphblas are all commutative? Or say that grouping does not matter?

Part of: https://github.com/pyOpenSci/software-submission/issues/81