patch 8.1.2304: cannot get the mouse position when getting a mouse click

Problem:    Cannot get the mouse position when getting a mouse click.
Solution:   Add getmousepos().
diff --git a/runtime/doc/popup.txt b/runtime/doc/popup.txt
index 39e4a3c..bdd8272 100644
--- a/runtime/doc/popup.txt
+++ b/runtime/doc/popup.txt
@@ -862,10 +862,8 @@
 	cursor keys	select another entry
 	Tab		accept current suggestion
 
-A mouse click arrives as <LeftMouse>.  The coordinates are in |v:mouse_col|
-and |v:mouse_lnum|.  |v:mouse_winid| holds the window ID, |v:mouse_win| is
-always zero.  The top-left screen cell of the popup is col 1, row 1 (not
-counting the border).
+A mouse click arrives as <LeftMouse>.  The coordinates can be obtained with
+|mousegetpos()|.
 
 Vim provides standard filters |popup_filter_menu()| and
 |popup_filter_yesno()|.