Describe the return statement (gpuweb#710)

* Describe the return statement

Also, in the grammar, the expression is optional.

* Updates from review

- Make a return_stmt grammar element
- Combine the return value paragraphs into one.

* Remove duplicate return_stmt grammar rule

wgsl/index.bs

@@ -995,9 +995,7 @@ statements
 
 statement
   : SEMICOLON
-  | RETURN logical_or_expression SEMICOLON
-      OpReturn
-      OpReturnValue
+  | return_stmt SEMICOLON
   | if_stmt
   | unless_stmt
   | switch_stmt
@@ -1306,6 +1304,25 @@ const_expr
   </xmp>
 </div>
 
+## Return statement ## {#return-statement}
+
+<pre>
+return_stmt
+  : RETURN logical_or_expression?
+</pre>
+
+A `return` statement ends execution of the current function.
+If the function is an entry point, then the current shader invocation
+is terminated.
+Otherwise, evaluation continues with the next expression or statement after
+the evaluation of the call site of the current function invocation.
+
+If the return type of the function is the void type, then the return statement
+must not have an expression.
+Otherwise the expression must be present, and is called the *return value*.
+In this case the call site of this function invocation evaluates to the return value.
+The type of the return value must match the return type of the function.
+
 # Validation # {#validation}
 
 Each validation item will be given a unique ID and a test must be provided