runtime(doc): add termdebug tag, remove term "floating window"

Problem:
- When I type `:h termdebug`, I will expect to see the introduction of
  the termdebug plugin. But instead, it shows me document of
  `termdebug_wide`, and I have to scroll up quite much to find the
  introduction.
- `:h popup` says `floating-window`? Why? As I have tried both features
  (of Vim and Neovim), I think they are _very different_ things, even
  more different than job features in Vim and Neovim.

Solution:
- In `:h terminal.txt`, add tag `*termdebug*` to the introduction of
  termdebug plugin.
- In `:h popup.txt`, "floating window" -> "popup window".

closes: #19135

Signed-off-by: Phạm Bình An <[email protected]>
Signed-off-by: Christian Brabandt <[email protected]>

Author: brianhuster

runtime/doc/popup.txt

@@ -1,10 +1,10 @@
-*popup.txt*	For Vim version 9.1.  Last change: 2025 Nov 09
+*popup.txt*	For Vim version 9.1.  Last change: 2026 Jan 08
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
 
 
-Displaying text in a floating window.	*popup* *popup-window* *popupwin*
+Displaying text in a popup window.	*popup* *popup-window* *popupwin*
 
 
 1. Introduction			|popup-intro|

runtime/doc/tags

@@ -10976,6 +10976,7 @@ termcap-cursor-shape	term.txt	/*termcap-cursor-shape*
 termcap-options	term.txt	/*termcap-options*
 termcap-title	term.txt	/*termcap-title*
 termda1-variable	eval.txt	/*termda1-variable*
+termdebug	terminal.txt	/*termdebug*
 termdebug-commands	terminal.txt	/*termdebug-commands*
 termdebug-communication	terminal.txt	/*termdebug-communication*
 termdebug-customizing	terminal.txt	/*termdebug-customizing*

runtime/doc/terminal.txt

@@ -1,4 +1,4 @@
-*terminal.txt*	For Vim version 9.1.  Last change: 2025 Nov 28
+*terminal.txt*	For Vim version 9.1.  Last change: 2026 Jan 08
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -1263,7 +1263,7 @@ Alternatively, press "s" to swap the first and second dump.  Do this several
 times so that you can spot the difference in the context of the text.
 
 ==============================================================================
-6. Debugging	*terminal-debug* *terminal-debugger* *package-termdebug*
+6. Debugging	*terminal-debug* *terminal-debugger* *package-termdebug* *termdebug*
 
 The Terminal debugging plugin can be used to debug a program with gdb and view
 the source code in a Vim window.  Since this is completely contained inside