math/big: document unsuitability for cryptography [1.20 backport]

[gopherbot]

@FiloSottile requested issue #58499 to be considered for backport to the next 1.20 minor release.

This was fixed by https://go.dev/cl/455135. @gopherbot please backport to Go 1.20 as it's an important documentation update that applies to the Go 1.20 release.

[gopherbot]

Change https://go.dev/cl/467835 mentions this issue: [release-branch.go1.20] math/big: add warning about using Int for cryptography

[heschi]

This backport seems to have been given up on. Closing.

cc @FiloSottile