Cross-compilation should document Python host version

When cross-compiling python binding, we rely on a `multistrap` generated chroot to build but we still need python on the host as well. Documentation should emphasize that in chroot and host version needs to match (up to major version), e.g., you need 3.7.x for building for a chroot 3.7.y, you can't have chroot using 3.7.y and host being 3.8.z.