patch 9.1.1123: popup hi groups not falling back to defaults

Problem:  Highlight groups PopupSelected/PopupNotification/
          MessageWindow are supposed to fall back to default highlight
          groups if they are not defined. However, once a colorscheme
          has defined them, switching to another colorscheme that
          doesn't do so will leave behind a cleared colorscheme, which
          causes the fallback to fail.

Solution: Set up default links to the relevant fallback highlight
          groups, which makes sure a `:hi clear` command will reset the
          state properly (Yee Cheng Chin).

closes: #16676

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/popup.txt b/runtime/doc/popup.txt
index 06acf39..ba87b31 100644
--- a/runtime/doc/popup.txt
+++ b/runtime/doc/popup.txt
@@ -1,4 +1,4 @@
-*popup.txt*  For Vim version 9.1.  Last change: 2025 Jan 08
+*popup.txt*  For Vim version 9.1.  Last change: 2025 Feb 20
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -504,7 +504,7 @@
 				\ mapping: 0,
 				\ })
 <		The current line is highlighted with a match using
-		"PopupSelected", or "PmenuSel" if that is not defined.
+		|hl-PopupSelected| which is linked to "PmenuSel" by default.
 
 		Use {options} to change the properties.  Should at least set
 		"callback" to a function that handles the selected item.
@@ -559,7 +559,7 @@
 				\ close: 'click',
 				\ padding: [0,1,0,1],
 				\ })
-<		The PopupNotification highlight group is used instead of
+<		The |hl-PopupNotification| highlight group is used instead of
 		WarningMsg if it is defined.
 
 		Without the |+timers| feature the popup will not disappear