Enable specifying directories in plugin-files.

doc/manual/command-ref/conf-file.xml

@@ -764,6 +764,10 @@ builtins.fetchurl {
         should not be linked to any Nix libs directly, as those will
         be available already at load time.
       </para>
+      <para>
+	If an entry in the list is a directory, all files in the
+	directory are loaded as plugins (non-recursively).
+      </para>
     </listitem>
 
   </varlistentry>