Fixes #3019 - Documentation tweak to default argument meaning.

documentation/index.html.erb

@@ -437,8 +437,8 @@ Expressions
     </p>
     <%= code_for('functions', 'cube(5)') %>
     <p>
-      Functions may also have default values for arguments. Override the default
-      value by passing a non-null argument.
+      Functions may also have default values for arguments, which will be used
+      if the incoming argument is missing (<tt>null</tt> or <tt>undefined</tt>).
     </p>
     <%= code_for('default_args', 'fill("cup")') %>