encoding/json: mention that the "string" tag applies to booleans

Fixes #9284

Change-Id: I0410a9ed82b861686a0a986c9ca4eeeacac8f296
Reviewed-on: https://go-review.googlesource.com/1534Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/json/encode.go

@@ -79,8 +79,8 @@ import (
 //
 // The "string" option signals that a field is stored as JSON inside a
 // JSON-encoded string. It applies only to fields of string, floating point,
-// or integer types. This extra level of encoding is sometimes used when
-// communicating with JavaScript programs:
+// integer, or boolean types. This extra level of encoding is sometimes used
+// when communicating with JavaScript programs:
 //
 //    Int64String int64 `json:",string"`
 //