spec: clarify use of built-ins in go/defer statements

1) Built-ins are restricted like for expression statements.
   This documents the status quo.

2) Calls cannot be parenthesized. The spec is not clear. gccgo
   permits it already, gc doesn't. Be explicit in the spec.

 Fixes #4462.

R=rsc, iant, r, ken, lvd
CC=golang-dev
https://golang.org/cl/6861043

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of November 26, 2012",
+	"Subtitle": "Version of November 29, 2012",
 	"Path": "/ref/spec"
 }-->
 
@@ -4431,7 +4431,7 @@ for w := range ch {
 <h3 id="Go_statements">Go statements</h3>
 
 <p>
-A "go" statement starts the execution of a function or method call
+A "go" statement starts the execution of a function call
 as an independent concurrent thread of control, or <i>goroutine</i>,
 within the same address space.
 </p>
@@ -4441,7 +4441,12 @@ GoStmt = "go" Expression .
 </pre>
 
 <p>
-The expression must be a call.
+The expression must be a function or method call; it cannot be parenthesized.
+Calls of built-in functions are restricted as for
+<a href="#Expression_statements">expression statements</a>.
+</p>
+
+<p>
 The function value and parameters are
 <a href="#Calls">evaluated as usual</a>
 in the calling goroutine, but
@@ -4758,7 +4763,12 @@ DeferStmt = "defer" Expression .
 </pre>
 
 <p>
-The expression must be a function or method call.
+The expression must be a function or method call; it cannot be parenthesized.
+Calls of built-in functions are restricted as for
+<a href="#Expression_statements">expression statements</a>.
+</p>
+
+<p>
 Each time the "defer" statement
 executes, the function value and parameters to the call are
 <a href="#Calls">evaluated as usual</a>