encoding/json: document that encoding.TextMarshaler is used if no (json) Marshaler is present

Change-Id: I63da54832548c325e47dc54aaa5b5112e1f3b3ba
Reviewed-on: https://go-review.googlesource.com/15048Reviewed-by: Rob Pike <r@golang.org>

src/encoding/json/encode.go

@@ -30,7 +30,10 @@ import (
 // Marshal traverses the value v recursively.
 // If an encountered value implements the Marshaler interface
 // and is not a nil pointer, Marshal calls its MarshalJSON method
-// to produce JSON.  The nil pointer exception is not strictly necessary
+// to produce JSON. If no MarshalJSON method is present but the
+// value implements encoding.TextMarshaler instead, Marshal calls
+// its MarshalText method.
+// The nil pointer exception is not strictly necessary
 // but mimics a similar, necessary exception in the behavior of
 // UnmarshalJSON.
 //