expvar: document that the Var interface's String method should return a valid JSON value.

Change-Id: If4e740f3dbef4053355542eebdd899b3099d872c
Reviewed-on: https://go-review.googlesource.com/21525
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/expvar/expvar.go

@@ -38,6 +38,7 @@ import (
 
 // Var is an abstract type for all exported variables.
 type Var interface {
+	// String returns a valid JSON value for the variable.
 	String() string
 }