patch 8.2.0385: menu functionality insufficiently tested

Problem:    Menu functionality insufficiently tested.
Solution:   Add tests.  Add menu_info(). (Yegappan Lakshmanan, closes #5760)

runtime/doc/usr_41.txt

@@ -942,10 +942,11 @@ Window size and position:			*window-size-functions*
 	winsaveview()		get view of current window
 	winrestview()		restore saved view of current window
 
-Mappings:				    *mapping-functions*
+Mappings and Menus:			    *mapping-functions*
 	hasmapto()		check if a mapping exists
 	mapcheck()		check if a matching mapping exists
 	maparg()		get rhs of a mapping
+	menu_info()		get information about a menu item
 	wildmenumode()		check if the wildmode is active
 
 Testing:				    *test-functions*