-This directive specifies a custom sidebar to display on the page, instead
-of any sidebar that is displayed globally.
+This directive can specify a custom sidebar to display on the page,
+overriding any sidebar that is displayed globally.
+
+If no custom sidebar content is specified, it forces the sidebar page to
+be used as the sidebar, even if the `global_sidebars` setting has been
+used to disable use of the sidebar page by default.