runtime: fix struct comment

Change-Id: I0c33830b13c8a187ac82504c7653abb8f8cf7530
Reviewed-on: https://go-review.googlesource.com/c/go/+/681655
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Sean Liao <sean@liao.dev>

src/runtime/mheap.go

@@ -312,8 +312,10 @@ type heapArena struct {
 	// during marking.
 	pageSpecials [pagesPerArena / 8]uint8
 
-	// pageUseSpanDartboard is a bitmap that indicates which spans are
-	// heap spans and also gcUsesSpanDartboard.
+	// pageUseSpanInlineMarkBits is a bitmap where each bit corresponds
+	// to a span, as only spans one page in size can have inline mark bits.
+	// The bit indicates that the span has a spanInlineMarkBits struct
+	// stored directly at the top end of the span's memory.
 	pageUseSpanInlineMarkBits [pagesPerArena / 8]uint8
 
 	// checkmarks stores the debug.gccheckmark state. It is only