go spec: fix and clarify syntax of conversions

Fixes #803.

R=rsc, r, iant, ken2
CC=golang-dev
https://golang.org/cl/1281041

doc/go_spec.html

@@ -1,5 +1,5 @@
 <!-- title The Go Programming Language Specification -->
-<!-- subtitle Version of May 14, 2010 -->
+<!-- subtitle Version of May 24, 2010 -->
 
 <!--
 Todo
@@ -3254,7 +3254,18 @@ that can be converted to type <code>T</code>.
 </p>
 
 <pre class="ebnf">
-Conversion = LiteralType "(" Expression ")" .
+Conversion = Type "(" Expression ")" .
+</pre>
+
+<p>
+If the type starts with an operator it must be parenthesized:
+</p>
+
+<pre>
+*Point(p)        // same as *(Point(p))
+(*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)
 </pre>
 
 <p>
@@ -3318,11 +3329,11 @@ string containing the UTF-8 representation of the integer.
 Negative values are converted to <code>"\uFFFD"</code>.
 
 <pre>
-string('a')  // "a"
+string('a')           // "a"
-string(-1)  // "\ufffd" == "\xef\xbf\xbd "
+string(-1)            // "\ufffd" == "\xef\xbf\xbd "
-string(0xf8)  // "\u00f8" == "ø" == "\xc3\xb8"
+string(0xf8)          // "\u00f8" == "ø" == "\xc3\xb8"
 type MyString string
-MyString(0x65e5)  // "\u65e5" == "日" == "\xe6\x97\xa5"
+MyString(0x65e5)      // "\u65e5" == "日" == "\xe6\x97\xa5"
 </pre>
 </li>