container/heap: avoid and/or ambiguity in documentation
container/heap: avoid and/or ambiguity in documentation

(per suggestion by Doug McIlroy)

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/50580046

wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang