Fix documentation for excluding templates in Sulu 2.x (#809)

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Co-authored-by: mcoiffier <[email protected]>

book/templates.rst

@@ -40,11 +40,10 @@ The template of a page can be selected in the admin interface:
 
 .. figure:: ../img/templates-selection.png
 
-.. Caution::
+.. note::
 
-    A template is shown in the dropdown only if both the XML and the Twig file
-    exist! If you can't see your template, double-check the directories
-    ``config/templates/pages`` and ``templates/pages``.
+    If you don't want a template to appear in this list, use the ``excluded-templates`` node in the webspace configuration file.
+    Have a closer look at `Setup a Webspace <webspaces.html#urls>`_ for more details.
 
 The name displayed in the dropdown is configured in the ``<meta>`` section of
 the XML: