Document return value of config_mgr_add_format().

src/app/config/confparse.c

@@ -203,6 +203,9 @@ config_mgr_register_fmt(config_mgr_t *mgr,
 /**
  * Add a new format to this configuration object.  Asserts on failure.
  *
+ * Returns an internal "index" value used to identify this format within
+ * all of those formats contained in <b>mgr</b>.  This index value
+ * should not generally be used outside of this module.
  **/
 int
 config_mgr_add_format(config_mgr_t *mgr,