doc: update function description for `decipher.setAAD`

According to the
[NodeJS CCM example](https://nodejs.org/docs/latest-v14.x/api/crypto.html#crypto_ccm_mode],
when decrypting the `plaintextLength` parameter actually refers to the
ciphertext length, not the plaintext length:

```
decipher.setAAD(aad, {
  plaintextLength: ciphertext.length
});
```

The same can be seen in the
[OpenSSL docs](https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption)
where a call to `EVP_DecryptUpdate` passes the ciphertext length:

```
/* Provide the total ciphertext length */
    if(1 != EVP_DecryptUpdate(ctx, NULL, &len, NULL, ciphertext_len))
        handleErrors();
```

This parameter probably should have been called `inputLength` or
`bufferLength` instead of `plaintextLength`, so that it makes sense
both when encrypting and decrypting, but at least we can correct the
sentence in the documentation for now to refer to the correct value.

PR-URL: https://github.com/nodejs/node/pull/33095


Reviewed-By: Ujjwal Sharma <ryzokuken@disroot.org>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>