spec: only require parens around ambiguous conversions

This is documenting the status quo. The previous cleanup
added this language as an implementation restriction, but
at least for now it is really part of the language proper.

Fixes #4605.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7305071

doc/go_spec.html

@@ -3448,16 +3448,20 @@ Conversion = Type "(" Expression [ "," ] ")" .
 
 <p>
 If the type starts with the operator <code>*</code> or <code>&lt;-</code>,
-or the keyword <code>func</code>, it must be parenthesized:
+or if the type starts with the keyword <code>func</code>
+and has no result list, it must be parenthesized when
+necessary to avoid ambiguity:
 </p>
 
 <pre>
 *Point(p)        // same as *(Point(p))
-(*Point)(p)      // p is converted to (*Point)
+(*Point)(p)      // p is converted to *Point
 &lt;-chan int(c)    // same as &lt;-(chan int(c))
-(&lt;-chan int)(c)  // c is converted to (&lt;-chan int)
+(&lt;-chan int)(c)  // c is converted to &lt;-chan int
 func()(x)        // function signature func() x
-(func())(x)      // x is converted to (func())
+(func())(x)      // x is converted to func()
+(func() int)(x)  // x is converted to func() int
+func() int(x)    // x is converted to func() int (unambiguous)
 </pre>
 
 <p>
@@ -3553,12 +3557,6 @@ implements this functionality under
 restricted circumstances.
 </p>
 
-<p>
-Implementation restriction: For backward-compatibility with the Go 1 language
-specification, a compiler may accept non-parenthesized literal function types
-in conversions where the syntax is unambiguous.
-</p>
-
 <h4>Conversions between numeric types</h4>
 
 <p>