math: make Hypot special cases look like others
math: make Hypot special cases look like others

The edit makes Hypot's description match the form
used in the other routines in this package.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/8003046
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go