Add some missing information to TabContentsObserver docs.

content/browser/tab_contents/tab_contents_observer.h

Index: content/browser/tab_contents/tab_contents_observer.h
===================================================================
--- content/browser/tab_contents/tab_contents_observer.h	(revision 86172)
+++ content/browser/tab_contents/tab_contents_observer.h	(working copy)
@@ -19,7 +19,9 @@
                             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);
@@ -124,6 +126,7 @@
  protected:
   friend class Registrar;
 
+  // Called from TabContents in response to having |this| added as an observer.
   void SetTabContents(TabContents* tab_contents);
 
  private: