runtime: clarify SetFinalizer docs

Fixes #24480

Change-Id: I7db721fb71a17f07472ec7f216478e7887435639
Reviewed-on: https://go-review.googlesource.com/122557Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mfinal.go

@@ -267,8 +267,8 @@ func runfinq() {
 // is not guaranteed to run, because there is no ordering that
 // respects the dependencies.
 //
-// The finalizer for obj is scheduled to run at some arbitrary time after
-// obj becomes unreachable.
+// The finalizer is scheduled to run at some arbitrary time after the
+// program can no longer reach the object to which obj points.
 // There is no guarantee that finalizers will run before a program exits,
 // so typically they are useful only for releasing non-memory resources
 // associated with an object during a long-running program.