[Sync EN] openssl: document algorithm = 0 and digest_alg => "null"

PR: `https://github.com/php/doc-en/pull/5878` (2026-09-25)

**Файлы RU для обновления**
- `reference/openssl/constants.xml`
- `reference/openssl/functions/openssl-csr-new.xml`
- `reference/openssl/functions/openssl-sign.xml`
- `reference/openssl/functions/openssl-verify.xml`

