[turbofan] Fix an incorrect comment, and elaborate.

src/compiler/node-marker.h

Index: src/compiler/node-marker.h
diff --git a/src/compiler/node-marker.h b/src/compiler/node-marker.h
index 5ef2063f18715cc5c59ddb9816f1de37c5ae2f1e..84666d5f076747d7e1be76ced39f8d7c10dd4c79 100644
--- a/src/compiler/node-marker.h
+++ b/src/compiler/node-marker.h
@@ -42,9 +42,22 @@ class NodeMarkerBase {
   DISALLOW_COPY_AND_ASSIGN(NodeMarkerBase);
 };
 
-
-// A NodeMarker uses monotonically increasing marks to assign local "states"
-// to nodes. Only one NodeMarker per graph is valid at a given time.
+// A NodeMarker assigns a local "state" to every node of a graph in constant
+// memory. Only one NodeMarker per graph is valid at a given time, that is,
+// after you create a NodeMarker you should no longer use NodeMarkers that
+// were created earlier. Internally, the local state is stored in the Node
+// structure.
+//
+// When you initialize a NodeMarker, all the local states are conceptually
+// set to State(0) in constant time.
+//
+// In its current implementation, in debug mode NodeMarker will try to
+// (efficiently) detect invalid use of an older NodeMarker. Namely, if you get
+// or set a node with a NodeMarker, and then get or set that node
+// with an older NodeMarker you will get a crash.
+//
+// GraphReducer uses a NodeMarker, so individual Reducers cannot use a
+// NodeMarker.
 template <typename State>
 class NodeMarker : public NodeMarkerBase {
  public: