patch 8.2.2296: cannot use CTRL-N and CTRL-P in a popup menu

Problem:    Cannot use CTRL-N and CTRL-P in a popup menu.
Solution:   Use CTRL-N like <Down> and CTRL-P like <Up>. (closes #7614)
diff --git a/runtime/doc/popup.txt b/runtime/doc/popup.txt
index be0b7e2..54302a9 100644
--- a/runtime/doc/popup.txt
+++ b/runtime/doc/popup.txt
@@ -309,8 +309,8 @@
 
 popup_filter_menu({id}, {key})				*popup_filter_menu()*
 		Filter that can be used for a popup. These keys can be used:
-		    j <Down>		select item below
-		    k <Up>		select item above
+		    j <Down> <C-N>	select item below
+		    k <Up> <C-P>	select item above
 		    <Space> <Enter>	accept current selection
 		    x Esc CTRL-C	cancel the menu
 		Other keys are ignored.