Add some missing information to TabContentsObserver docs.

content/browser/tab_contents/tab_contents_observer.h

 // Copyright (c) 2011 The Chromium 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 CONTENT_BROWSER_TAB_CONTENTS_TAB_CONTENTS_OBSERVER_H_
 #define CONTENT_BROWSER_TAB_CONTENTS_TAB_CONTENTS_OBSERVER_H_
 
 #include "content/browser/tab_contents/navigation_controller.h"
 #include "content/common/page_transition_types.h"
 #include "ipc/ipc_channel.h"
 #include "webkit/glue/window_open_disposition.h"
 
 class RenderViewHost;
 struct ViewHostMsg_FrameNavigate_Params;
 
 // An observer API implemented by classes which are interested in various page
 // load events from TabContents.  They also get a chance to filter IPC messages.
 class TabContentsObserver : public IPC::Channel::Listener,
                             public IPC::Message::Sender {
  public:
   // Use this as a member variable in a class that uses the empty constructor
-  // version of this interface.
+  // version of this interface.  On destruction of TabContents being observed,
+  // the registrar must either be destroyed or explicitly set to observe
+  // another TabContents.
   class Registrar {
    public:
     explicit Registrar(TabContentsObserver* observer);
     ~Registrar();
 
     // Call this to start observing a tab.  Passing in NULL resets it.
     // This can only be used to watch one tab at a time.  If you call this and
     // you're already observing another tab, the old tab won't be observed
     // afterwards.
     void Observe(TabContents* tab);
@@ skipping 84 matching lines @@
 
   // IPC::Message::Sender implementation.
   virtual bool Send(IPC::Message* message);
 
   TabContents* tab_contents() const { return tab_contents_; }
   int routing_id() const;
 
  protected:
   friend class Registrar;
 
+  // Called from TabContents in response to having |this| added as an observer.
   void SetTabContents(TabContents* tab_contents);
 
  private:
   friend class TabContents;
 
   // Invoked from TabContents. Invokes TabContentsDestroyed and NULL out
   // |tab_contents_|.
   void TabContentsDestroyed();
 
   TabContents* tab_contents_;
 
   // The routing ID of the associated TabContents.
   int routing_id_;
 
   DISALLOW_COPY_AND_ASSIGN(TabContentsObserver);
 };
 
 #endif  // CONTENT_BROWSER_TAB_CONTENTS_TAB_CONTENTS_OBSERVER_H_