Specify module __init function

Fixes #101.

lang/spec.html

@@ -5365,6 +5365,16 @@ <h3>Function definition</h3>
 An <code>external-function-body</code> means that the implementation of the
 function is not provided in the Ballerina source module.
 </p>
+<p>
+If a module has a function-defn with an identifier of <code>__init</code>, it is
+called called automatically by the system at the end of the initialization of
+that module; if this call returns an error, then initialization of the module
+fails. The following special requirements apply to the <code>__init</code>
+function of a module: it must not be declared <code>public</code>; its return
+type must both be a subtype of <code>error?</code> and contain <code>()</code>;
+it must have no parameters.
+</p>
+
 <h3>Service declaration</h3>
 
 <pre