Document the significance of the trailing slash in elements of

completion-ignored-extensions.

etc/NEWS

@@ -26,6 +26,13 @@ much pure storage it will approximately need.
 `${foo}' in the specified new value of the environment variable.  To
 include a `$' in the value, use `$$'.
 
+** File-name completion can now ignore directories.
+If an element of the list in `completion-ignored-extensions' ends in a
+slash `/', it indicates a subdirectory that should be ignored when
+completing file names.  Elements of `completion-ignored-extensions'
+which do not end in a slash are never considered when a completion
+candidate is a directory.
+
 ** New user option `inhibit-startup-buffer-menu'.
 When loading many files, for instance with `emacs *', Emacs normally
 displays a buffer menu.  This option turns the buffer menu off.