[turbofan] Fix an incorrect comment, and elaborate.

src/compiler/node-marker.h

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_COMPILER_NODE_MARKER_H_
 #define V8_COMPILER_NODE_MARKER_H_
 
 #include "src/compiler/node.h"
 
 namespace v8 {
@@ skipping 24 matching lines @@
     node->set_mark(mark + mark_min_);
   }
 
  private:
   Mark const mark_min_;
   Mark const mark_max_;
 
   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:
   V8_INLINE NodeMarker(Graph* graph, uint32_t num_states)
       : NodeMarkerBase(graph, num_states) {}
 
   V8_INLINE State Get(Node* node) {
     return static_cast<State>(NodeMarkerBase::Get(node));
   }
 
   V8_INLINE void Set(Node* node, State state) {
     NodeMarkerBase::Set(node, static_cast<Mark>(state));
   }
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_NODE_MARKER_H_