patch 9.1.0878: termdebug: cannot enable DEBUG mode

Problem:  termdebug: cannot enable DEBUG mode
Solution: Allow to specify DEBUG mode (Ubaldo Tiberi)

closes: #16080

Signed-off-by: Ubaldo Tiberi <ubaldo.tiberi@volvo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/tags

@@ -10688,6 +10688,7 @@ termdebug-starting	terminal.txt	/*termdebug-starting*
 termdebug-stepping	terminal.txt	/*termdebug-stepping*
 termdebug-timeout	terminal.txt	/*termdebug-timeout*
 termdebug-variables	terminal.txt	/*termdebug-variables*
+termdebug_contributing	terminal.txt	/*termdebug_contributing*
 termdebug_disasm_window	terminal.txt	/*termdebug_disasm_window*
 termdebug_evaluate_in_popup	terminal.txt	/*termdebug_evaluate_in_popup*
 termdebug_map_K	terminal.txt	/*termdebug_map_K*

runtime/doc/terminal.txt

@@ -1,4 +1,4 @@
-*terminal.txt*	For Vim version 9.1.  Last change: 2024 Nov 10
+*terminal.txt*	For Vim version 9.1.  Last change: 2024 Nov 19
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -1737,4 +1737,23 @@ This can also be used in a "one-shot" manner: >
 	  let g:termdebug_config['evaluate_in_popup'] = v:false
 	endfunc
 <
+
+Contributing ~
+						*termdebug_contributing*
+Contributions for termdebug improvements are welcome.
+However, it is fairly common that during the development process you need some
+mechanisms like `echo` statements (or similar) to help you in your job.
+For this reason, you can set: >
+    let g:termdebug_config['debug'] = true
+<
+This sets the `DEBUG` variable to `true` in the source code that you can use
+within the source code. An example of its usage follows: >
+    if exists('g:termdebug_loaded')
+      if DEBUG
+	Echoerr('Termdebug already loaded.')
+      endif
+      finish
+    endif
+<
+
  vim:tw=78:ts=8:noet:ft=help:norl: