Doc: Describe "Ask for confirmation before exiting" option

Update screenshot of the Environment > System tab that
contains the new option.

Task-number: QTCREATORBUG-24529
Change-Id: Ia8a3f048ee492b2a5d37138b2d82638b5bf91c6f
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

doc/qtcreator/src/howto/creator-keyboard-shortcuts.qdoc

@@ -283,6 +283,12 @@
             \li  Esc
         \row
             \li  Exit \QC
+
+                 By default, \QC exits without asking for confirmation, unless
+                 there are unsaved changes in open files. To always be asked,
+                 select the \uicontrol {Ask for confirmation before exiting}
+                 check box in \uicontrol Tools > \uicontrol Options >
+                 \uicontrol Environment > \uicontrol System.
             \li  Ctrl+Q
     \endtable