builtin: document true and false.

Fixes #5315.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8841048

src/pkg/builtin/builtin.go

@@ -13,6 +13,12 @@ package builtin
 // bool is the set of boolean values, true and false.
 type bool bool
 
+// true and false are the two untyped boolean values.
+const (
+	true  = 0 == 0 // Untyped bool.
+	false = 0 != 0 // Untyped bool.
+)
+
 // uint8 is the set of all unsigned 8-bit integers.
 // Range: 0 through 255.
 type uint8 uint8