Update _st.js

Added description of _st function. See email exchange with Herby Fri, Sep 6, 2013 at 11:28 AM

support/_st.js

@@ -1,3 +1,15 @@
+
+/* 
+
+_st points to a function used all over the compiled amber code that
+takes any value (JavaScript or Smalltalk) and returns a proper Amber Smalltalk receiver.
+
+null or undefined -> nil,
+plain JS object -> wrapped JS object,
+otherwise unchanged
+
+*/
+
 define("amber_vm/_st", ["./boot"], function (boot) {
     return boot._st;
 });