Update docs/tests/navigation/exclude/untitled.md

Co-authored-by: Peter Mosses <18308236+pdmosses@users.noreply.github.com>

docs/tests/navigation/exclude/untitled.md

@@ -3,4 +3,4 @@ layout: default
 ---
 # Untitled
 
-This page has no `title`, and should not appear in the navigation.
+This page does not have a `title`, and it is excluded from the navigation -- unless it is located in a Jekyll collection (which provides default titles). To exclude a title-less page from the navigation, regardless of whether it is located in a collection, set `nav_exclude: true`.