C: add Hana Kim (Google CLA)

LGTM=crawshaw
R=golang-codereviews, crawshaw
CC=golang-codereviews, iant
https://codereview.appspot.com/150690043
runtime: add comment to mgc0.h
Missed that comment in CL 153990043.

LGTM=khr
R=khr
CC=golang-codereviews
https://codereview.appspot.com/156010043