patch 8.1.1875: cannot get size and position of the popup menu

Problem:    Cannot get size and position of the popup menu.
Solution:   Add pum_getpos(). (Ben Jackson, closes #4827)
diff --git a/runtime/doc/autocmd.txt b/runtime/doc/autocmd.txt
index 0cab9d8..7893651 100644
--- a/runtime/doc/autocmd.txt
+++ b/runtime/doc/autocmd.txt
@@ -595,6 +595,10 @@
 				    scrollbar		TRUE if visible
 
 				It is not allowed to change the text |textlock|.
+
+				The size and position of the popup are also
+				available by calling |pum_getpos()|.
+
 							*CompleteDone*
 CompleteDone			After Insert mode completion is done.  Either
 				when something was completed or abandoning