Currently the doc string forms one run-on sentence when viewed in
a browser. Add an additional newline so the text is presented in
a browser the same way it is in the source code.

Change-Id: If93e45c5aec9edd195c6bf36cb757c5146368507
Reviewed-on: https://go-review.googlesource.com/101284Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>