More thorough documentation and features for random (multivariate) polynomials

[albinahlback]

I wanted to estimate some computational times for a project, and I wanted to generate some random multivariate polynomials of different shapes. Now, AbstractAlgebra does seem to support some way of constructing random multivariate polynomials, but I cannot get it to work for ZZMPolyRing.

Moreover, no characteristics of the multivariate polynomial can be given (say, square-free), or the distribution of zero coefficients, and I find the documentation to be poor.

Can this be improved?

[lgoettgens]

x-ref: #2896

[albinahlback]

Ah, I tried to search for a similar issue but I did not find it. Thanks!