runtime: expand comment on work.spans

Change-Id: I4b8a6f5d9bc5aba16026d17f99f3512dacde8d2d
Reviewed-on: https://go-review.googlesource.com/30534
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -815,7 +815,9 @@ var work struct {
 	// mode is the concurrency mode of the current GC cycle.
 	mode gcMode
 
-	// Copy of mheap.allspans for marker or sweeper.
+	// Snapshot of mheap.allspans for marker or sweeper created by
+	// gcCopySpans. Unlike allspans, this does not change except
+	// at controlled GC transition points.
 	spans []*mspan
 
 	// totaltime is the CPU nanoseconds spent in GC since the