runtime: add comment to mgc0.h Missed that comment in CL 153990043.
runtime: add comment to mgc0.h
Missed that comment in CL 153990043.

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

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go