[Sync EN] openssl: document algorithm = 0 and digest_alg => "null" (#5878)

Commit: `https://github.com/php/doc-en/commit/839637661508bf6e52d49061b060579e6a12fe52`
PR: `https://github.com/php/doc-en/pull/5878`

**Archivos ES a actualizar**
- `reference/openssl/constants.xml`
- `reference/openssl/functions/openssl-csr-new.xml`
- `reference/openssl/functions/openssl-sign.xml`
- `reference/openssl/functions/openssl-verify.xml`

