document FunctionType in C#

doc/runtime-api.html

@@ -886,6 +886,10 @@ Det -> CN -> NP
 System.out.println(gr.getFunctionType("DetCN"));
 Det -> CN -> NP
 </pre>
+<pre class="csharp">
+Console.WriteLine(gr.FunctionType("DetCN"));
+Det -> CN -> NP
+</pre>
 
 <h2>Type Checking Abstract Trees</h2>