ocaml-crypto/rng: rename crypto-rng.unix to nox-crypto-rng.unix in error text
The package was renamed in 46ba2cfe7 but the unseeded-generator and
no-default-generator panic messages still pointed users at the upstream
'crypto-rng.unix' name. Update both messages and the doc reference in
crypto_rng.mli to the post-rename name. Drop the
'auto-initializes on load' phrasing since the unix sublib no longer
auto-runs at module load — callers must invoke Crypto_rng_unix.use_default
explicitly.