Add comment on impl safety

src/lib/vm/objects/block.rs

@@ -74,11 +74,14 @@ pub struct Block {
     pub method_stack_base: usize,
 }
 
-// By default, `Block` is not `FinalizerSafe` because it has fields to other
-// `Gc`s. These fields would be unsound to access inside a `drop` method, so
-// this explicit impl is needed to tell the compiler we are not doing that.
+// The following impls are needed to allow the GC to finalise `Block`. By
+// default, `Block` is not `FinalizerSafe` because it has fields to other `Gc`s.
+// These fields would be unsound to access inside a `drop` method, so this
+// explicit impl is needed to tell the compiler we are not doing that. The only
+// time a `Block` is shared between threads is inside a finaliser, which is
+// Sync-safe because we guarantee it will not be accessed at the same time as
+// the main-thread.
 unsafe impl FinalizerSafe for Block {}
-
 unsafe impl Sync for Block {}
 
 impl Obj for Block {