spec: BuiltinCall syntax must permit "..." for append

Also: fix an incorrect link.

Fixes #4479.
Fixes #4456.

R=rsc, iant, r, ken
CC=golang-dev
https://golang.org/cl/6868062

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of December 3, 2012",
+	"Subtitle": "Version of December 4, 2012",
 	"Path": "/ref/spec"
 }-->
 
@@ -1013,7 +1013,7 @@ promoted methods are included in the method set of the struct as follows:
 A field declaration may be followed by an optional string literal <i>tag</i>,
 which becomes an attribute for all the fields in the corresponding
 field declaration. The tags are made
-visible through a <a href="#Package_unsafe">reflection interface</a>
+visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
 but are otherwise ignored.
 </p>
 
@@ -4831,7 +4831,7 @@ they cannot be used as function values.
 
 <pre class="ebnf">
 BuiltinCall = identifier "(" [ BuiltinArgs [ "," ] ] ")" .
-BuiltinArgs = Type [ "," ExpressionList ] | ExpressionList .
+BuiltinArgs = Type [ "," ArgumentList ] | ArgumentList .
 </pre>
 
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