Fix docstring for fmpz_poly_is_squarefree
Solves #2084
@fingolfin
@edgarcosta please make a suggestion here if you want one
I honestly find it confusing that the polynomial zero is square-free, but perhaps a change in the code now is too late.
I don't know. If people have a problem with it, I'm down for changing it.
@fredrik-johansson would it be okay to change the definition of square free?
I'd be happy with 0 being considered not squarefree.
FWIW we came to the same conclusion for Nemo, see https://github.com/Nemocas/Nemo.jl/issues/1510
Seems like a reasonable fix before releasing 3.2.0. However, we would have to make sure that this is consistent throughout FLINT. Moreover, it seems like some docstring do not provide the definition of square-free (see docstring for fq_poly_is_squarefree).