math/big: document unsuitability for cryptography

[FiloSottile]

We mentioned in the Go 1.20 release notes that following our own migration off math/big for cryptography (#52182), the package makes an especially bad choice for cryptographic use. The package docs should mention that too.

[FiloSottile]

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]

Backport issue(s) opened: #58500 (for 1.20).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[gopherbot]

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