runtime: fix out-of-date comment doc

Change-Id: I352fa0e4e048b896d63427f1c2c519bfed24c702
Reviewed-on: https://go-review.googlesource.com/c/go/+/622017
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/mbarrier.go

@@ -92,19 +92,6 @@ import (
 // barriers, which will slow down both the mutator and the GC, we always grey
 // the ptr object regardless of the slot's color.
 //
-// Another place where we intentionally omit memory barriers is when
-// accessing mheap_.arena_used to check if a pointer points into the
-// heap. On relaxed memory machines, it's possible for a mutator to
-// extend the size of the heap by updating arena_used, allocate an
-// object from this new region, and publish a pointer to that object,
-// but for tracing running on another processor to observe the pointer
-// but use the old value of arena_used. In this case, tracing will not
-// mark the object, even though it's reachable. However, the mutator
-// is guaranteed to execute a write barrier when it publishes the
-// pointer, so it will take care of marking the object. A general
-// consequence of this is that the garbage collector may cache the
-// value of mheap_.arena_used. (See issue #9984.)
-//
 //
 // Stack writes:
 //