document that the page.title is in HTML format

docs/modules/ROOT/pages/templates.adoc

@@ -65,7 +65,8 @@ s| [[page]]page
 | Information about the current page.
 
 | page.title
-| The page title (also used as the primary heading).
+| The page title in HTML format (often used as the primary heading).
+This value may include inline HTML elements and XML character references.
 
 | page.contents
 | The main article content in HTML format.