Reword the menu paragraph

README.md

@@ -81,9 +81,11 @@ the section menus, if any; the page title, if any; the page description,
 if any; and the page date, if any. The body has the page content, if
 any. The footer has the site copyright notice, if any.
 
-The section menus are activated if their path matches the current page.
+A section menu is highlighted if its path is a prefix of the path of the
-Pages in the directory `content/` match the section path `/`. Pages in
+current page. For example, the section menu path `/foo/` matches the
-the directory `content/foo/` match the section path `/foo/`.
+pages `content/foo/_index.md`, `content/foo/bar.md`, and
+`content/foo/bar/_index.md`. The section menu path `/` only matches the
+page `content/_index.md`.
 
 The page date is the publish date, if any; otherwise, it's the creation
 date.