Document that factorisation of univariate polynomials over integers mod n only works correctly if $n$ is a prime

[fingolfin]

This should be mentioned in the documentation but isn't, see https://flintlib.org/doc/nmod_poly_factor.html

Cf. Nemocas/Nemo.jl#2253

[albinahlback]

Good point. @fredrik-johansson should we also put in a couple of FLINT_ASSERT(n_is_prime(poly->mod.n)) at places in the module?

[fingolfin]

Do these asserts result in a n error value being returned (fine), or in a crash?

[albinahlback]

It would result in a crash, but only if it is compiled with --enable-assert (which, FLINT_jll is not compiled with).