Rearrange return-types

Prepare for #426

lang/spec.html

@@ -1745,22 +1745,14 @@ <h4 id="functions">Functions</h4>
 <pre
 class="grammar">function-type-descriptor := <code>function</code> function-signature
 function-signature := <code>(</code> param-list <code>)</code> return-type-descriptor
-return-type-descriptor := [ <code>returns</code> [annots] type-descriptor ]
 </pre>
 <p>
 A function is a part of a program that can be explicitly executed. In Ballerina,
 a function is also a value, implying that it can be stored in variables, and
 passed to or returned from functions. When a function is executed, it is passed
 an argument list as input and returns a value as output.
 </p>
-<p>
-When the execution of a function returns to its caller, it returns exactly one
-value. A function that would in other programming languages not return a value
-is represented in Ballerina by a function returning <code>()</code>. Note that
-the function definition does not have to explicitly return <code>()</code>; a
-return statement or falling off the end of the function body will implicitly
-return <code>()</code>.
-</p>
+
 <pre
 class="grammar">param-list :=
    required-params [<code>,</code> defaultable-params] [<code>,</code> rest-param]
@@ -1847,6 +1839,22 @@ <h4 id="functions">Functions</h4>
 make use of arguments specified both by position and by name, is described in
 more detail in the <a href="#function_call">section on function calls</a>.
 </p>
+<pre
+class="grammar">return-type-descriptor := [ <code>returns</code> [annots] type-descriptor ]
+</pre>
+<p>
+When the execution of a function returns to its caller, it returns exactly one
+value. A function that would in other programming languages not return a value
+is represented in Ballerina by a function returning <code>()</code>. Note that
+the function definition does not have to explicitly return <code>()</code>; a
+return statement or falling off the end of the function body will implicitly
+return <code>()</code>.
+</p>
+<p>
+The value returned by a function will belong to the type specified in the
+return-type-descriptor. An empty return-type-descriptor is equivalent to
+<code>returns ()</code>.
+</p>
 <p>
 Function types are covariant in their return types and contravariant in the type
 of their parameter lists. More precisely, a function type with return type R and