Add comment explaining finish output size

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

library/psa_crypto_aead.c

@@ -677,6 +677,8 @@ psa_status_t mbedtls_psa_aead_finish(
 
     if( status == PSA_SUCCESS )
     {
+        /* This will be zero for all supported algorithms currently, but left
+         * here for future support. */
         *ciphertext_length = finish_output_size;
         *tag_length = operation->tag_length;
     }