patch 9.0.0321: cannot use the message popup window directly

Problem:    Cannot use the message popup window directly.
Solution:   Add ":echowindow".
diff --git a/src/structs.h b/src/structs.h
index 490a973..b8627dd 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2606,7 +2606,7 @@
     POPPOS_BOTRIGHT,
     POPPOS_TOPRIGHT,
     POPPOS_CENTER,
-    POPPOS_BOTTOM,	// bottom of popup at bottom of screen
+    POPPOS_BOTTOM,	// bottom of popup just above the command line
     POPPOS_NONE
 } poppos_T;