document exact BigInt determinants (#53579)
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.)
