runtime: improve comment in concurrent GC

"Sync" is not very informative. What's being synchronized and with
whom? Update this comment to explain what we're really doing: enabling
write barriers.

Change-Id: I4f0cbb8771988c7ba4606d566b77c26c64165f0f
Reviewed-on: https://go-review.googlesource.com/7700Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -333,7 +333,7 @@ func gc(mode int) {
 			gcscan_m()
 			gctimer.cycle.installmarkwb = nanotime()
 
-			// Sync.
+			// Enter mark phase and enable write barriers.
 			stoptheworld()
 			gcphase = _GCmark