doc: mention WriteHeapDump in 1.3 release notes
doc: mention WriteHeapDump in 1.3 release notes

LGTM=r
R=khr, r
CC=golang-codereviews
https://codereview.appspot.com/103810044

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang