Improve comments

library/x509_crt.c

@@ -2310,7 +2310,9 @@ int mbedtls_x509_crt_verify_with_profile( mbedtls_x509_crt *crt,
     }
 
 exit:
-    /* prevent misuse of the vrfy callback */
+    /* prevent misuse of the vrfy callback - VERIFY_FAILED would be ignored by
+     * the SSL module for authmode optional, but non-zero return from the
+     * callback means a fatal error so it shouldn't be ignored */
     if( ret == MBEDTLS_ERR_X509_CERT_VERIFY_FAILED )
         ret = MBEDTLS_ERR_X509_FATAL_ERROR;