go/doc: add nil to list of predeclared constants

R=gri
CC=gobot, golang-dev
https://golang.org/cl/6601054

src/pkg/go/doc/example.go

@@ -138,7 +138,7 @@ func playExample(file *ast.File, body *ast.BlockStmt) *ast.File {
 
 	// Remove predeclared identifiers from unresolved list.
 	for n := range unresolved {
-		if n == "nil" || predeclaredTypes[n] || predeclaredConstants[n] || predeclaredFuncs[n] {
+		if predeclaredTypes[n] || predeclaredConstants[n] || predeclaredFuncs[n] {
 			delete(unresolved, n)
 		}
 	}

src/pkg/go/doc/reader.go

@@ -751,7 +751,7 @@ func sortedFuncs(m methodSet, allMethods bool) []*Func {
 }
 
 // ----------------------------------------------------------------------------
-// Predeclared identifiers (minus "nil")
+// Predeclared identifiers
 
 var predeclaredTypes = map[string]bool{
 	"bool":       true,
@@ -795,7 +795,8 @@ var predeclaredFuncs = map[string]bool{
 }
 
 var predeclaredConstants = map[string]bool{
+	"false": true,
 	"iota":  true,
+	"nil":   true,
 	"true":  true,
-	"false": true,
 }