Int.String had no documentation and the documentation for Int.Text
did not mention the handling of the nil pointer case.

Change-Id: I9f21921e431c948545b7cabc7829e4b4e574bbe9
Reviewed-on: https://go-review.googlesource.com/c/go/+/175118Reviewed-by: Robert Griesemer <gri@golang.org>