document exact BigInt determinants

[stevengj]

New users are commonly surprised that determinants of integer matrices give an approximate floating-point answer (#40128), and are unaware that an exact algorithm is implemented for BigInt matrices (#40868). This PR comments on both of these facts in the det documentation.

(At some point, we may want to mark LinearAlgebra.det_bareiss as public, and document it, but that can be done in a future PR.)

[jmichel7]

Something worth documenting with det_bareiss is exactdiv. This routine is fundamental for making this kind of linear algebra to work over a ring.

For instance, it is sufficient to define exactdiv for polynomials to have det_bareiss work for a matrix of polynomials. I have implemented quite a few similar linear algebra algorithms over a ring where exactdiv is useful in the same way, for instance computing the characteristic polynomial or the comatrix (the inverse times the determinant).

[stevengj]

Yes, but documenting det_bareiss is a topic for another issue, as I mentioned above — I agree that one would want to document exactdiv as well in that case. This PR is just focused on documenting det.