spec: an initial BOM can be ignored

After further deliberation, let's back down to the Unicode proposal.
Ignoring aBOMinations anywhere means that things like
        grep unsafe *.go
might fail because there's a BOM in the middle: unBOMsafe.

R=golang-dev, rsc, 0xjnml, gri, bradfitz
CC=golang-dev
https://golang.org/cl/6490091

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of September 6, 2012",
+	"Subtitle": "Version of September 7, 2012",
 	"Path": "/ref/spec"
 }-->
 
@@ -101,9 +101,8 @@ compiler may disallow the NUL character (U+0000) in the source text.
 </p>
 <p>
 Implementation restriction: For compatibility with other tools, a
-compiler may ignore any UTF-8-encoded Unicode byte order mark
-(U+FEFF) in the source text outside of <a href="#String_literals">string</a>
-and <a href="#Rune_literals">rune</a> literals.
+compiler may ignore a UTF-8-encoded byte order mark
+(U+FEFF) if it is the first Unicode code point in the source text.
 </p>
 
 <h3 id="Characters">Characters</h3>