patch 8.2.1961: various comments can be improved

Problem:    Various comments can be improved.
Solution:   Various comment adjustments.
diff --git a/src/time.c b/src/time.c
index cc29d0d..d1cf21b 100644
--- a/src/time.c
+++ b/src/time.c
@@ -497,10 +497,12 @@
 	    may_garbage_collect = FALSE;
 	    save_vimvars(&vvsave);
 
+	    // Invoke the callback.
 	    timer->tr_firing = TRUE;
 	    timer_callback(timer);
 	    timer->tr_firing = FALSE;
 
+	    // Restore stuff.
 	    timer_next = timer->tr_next;
 	    did_one = TRUE;
 	    timer_busy = save_timer_busy;