Another comment update.

I thought this was in my last commit, but I goofed.

src/backend/postmaster/pgstat.c

@@ -2905,8 +2905,8 @@ pgstat_progress_update_param(int index, int64 val)
 /*-----------
  * pgstat_progress_update_multi_param() -
  *
- * Automatically update multiple members in st_progress_param[] of own backend
- * entry.
+ * Update multiple members in st_progress_param[] of own backend entry.
+ * This is atomic; readers won't see intermediate states.
  *-----------
  */
 void