Clarifying that FindString only provides left-most match

Change-Id: Ic6ecec12cca759fd4b3565ef5901a110843ffd56
Reviewed-on: https://go-review.googlesource.com/48609Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>