runtime: add comment to mgc0.h

Missed that comment in CL 153990043.

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/156010043

Author: dvyukov

src/runtime/mgc0.h

@@ -42,6 +42,8 @@ enum {
 	BitsMask	= (1<<BitsPerPointer)-1,
 	PointersPerByte	= 8/BitsPerPointer,
 
+	// If you change these, also change scanblock.
+	// scanblock does "if(bits == BitsScalar || bits == BitsDead)" as "if(bits <= BitsScalar)".
 	BitsDead	= 0,
 	BitsScalar	= 1,
 	BitsPointer	= 2,