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/syntax.txt b/runtime/doc/syntax.txt
index 38f764f..cb6704c 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -1,4 +1,4 @@
-*syntax.txt*	For Vim version 9.1.  Last change: 2025 Jan 20
+*syntax.txt*	For Vim version 9.1.  Last change: 2025 Feb 20
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -5831,8 +5831,8 @@
 MatchParen	Character under the cursor or just before it, if it
 		is a paired bracket, and its match. |pi_paren.txt|
 							*hl-MessageWindow*
-MessageWindow	Messages popup window used by `:echowindow`.  If not defined
-		|hl-WarningMsg| is used.
+MessageWindow	Messages popup window used by `:echowindow`.  Linked to
+		|hl-WarningMsg| by default.
 							*hl-ModeMsg*
 ModeMsg		'showmode' message (e.g., "-- INSERT --").
 							*hl-MsgArea*
@@ -5872,10 +5872,13 @@
 		combination with |hl-PmenuSel|.
 							*hl-ComplMatchIns*
 ComplMatchIns	Matched text of the currently inserted completion.
+							*hl-PopupSelected*
+PopupSelected	Popup window created with |popup_menu()|.  Linked to
+		|hl-PmenuSel| by default.
 							*hl-PopupNotification*
 PopupNotification
-		Popup window created with |popup_notification()|.  If not
-		defined |hl-WarningMsg| is used.
+		Popup window created with |popup_notification()|.  Linked to
+		|hl-WarningMsg| by default.
 							*hl-Question*
 Question	|hit-enter| prompt and yes/no questions.
 							*hl-QuickFixLine*