math/cmplx: specify which square root Sqrt returns
math/cmplx: specify which square root Sqrt returns

Fixes issue 7851.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/93200043

pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go