Doc: Make the instructions for enabling plugins more consistent

- Add the following for documented plugins that are
  disabled by default: \note Enable the <name> plugin to use it
- Add \sa {Enable and disable plugins} for the above plugins and
  describe there how to find the About Plugins dialog on macOS
- For the one occurrence in the QDS Manual, add instructions for
  finding the About Plugins dialog on macOS ("Enable and disable plugins"
  is not included in the QDS Manual
- Mostly remove other kinds of references to enabling plugins

Task-number: QTCREATORBUG-29734
Change-Id: I7b12bd651c1c99e0c6b3ca6f94d3831ebe038438
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

doc/qtcreator/src/user-interface/creator-only/creator-reference-to-do-entries-view.qdoc

@@ -9,6 +9,8 @@
 
     \title To-Do Entries
 
+    \note Enable the Todo plugin to use it.
+
     \brief Lists the BUG, FIXME, NOTE, TODO, and WARNING keywords from the
     current file, from all project files, or from a subproject.
 
@@ -21,17 +23,6 @@
     analysis tools in \l Issues. For more information, see
     \l{Show task list files in Issues}.
 
-    \section1 Enabling the To-Do Plugin
-
-    The Todo plugin is disabled by default. To enable it:
-
-    \list 1
-        \li Select \uicontrol Help > \uicontrol {About Plugins} >
-            \uicontrol Utilities > \uicontrol Todo.
-        \li Select \uicontrol OK.
-        \li Select \uicontrol {Restart Now} to restart \QC and load the plugin.
-    \endlist
-
     \section1 To-Do Preferences
 
     To add keywords, select \preferences >