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: #33095
Reviewed-By: Ujjwal Sharma <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>

doc/api/crypto.md

@@ -493,7 +493,7 @@ _additional authenticated data_ (AAD) input parameter.
 
 The `options` argument is optional for `GCM`. When using `CCM`, the
 `plaintextLength` option must be specified and its value must match the length
-of the plaintext in bytes. See [CCM mode][].
+of the ciphertext in bytes. See [CCM mode][].
 
 The `decipher.setAAD()` method must be called before [`decipher.update()`][].
 
@@ -3088,7 +3088,12 @@ mode must adhere to certain restrictions when using the cipher API:
   mode might fail as CCM cannot handle more than one chunk of data per instance.
 * When passing additional authenticated data (AAD), the length of the actual
   message in bytes must be passed to `setAAD()` via the `plaintextLength`
-  option. This is not necessary if no AAD is used.
+  option.
+  Many crypto libraries include the authentication tag in the ciphertext,
+  which means that they produce ciphertexts of the length
+  `plaintextLength + authTagLength`. Node.js does not include the authentication
+  tag, so the ciphertext length is always `plaintextLength`.
+  This is not necessary if no AAD is used.
 * As CCM processes the whole message at once, `update()` can only be called
   once.
 * Even though calling `update()` is sufficient to encrypt/decrypt the message,