Document that wxString::Clone() is obsolete

It's useless now that we never use ref-counted implementation any
longer.

include/wx/string.h

@@ -1298,8 +1298,6 @@ public:
 
   wxString Clone() const
   {
-      // make a deep copy of the string, i.e. the returned string will have
-      // ref count = 1 with refcounted implementation
       return wxString::FromImpl(wxStringImpl(m_impl.c_str(), m_impl.length()));
   }
 

interface/wx/string.h

@@ -1497,14 +1497,12 @@ public:
     bool Shrink();
 
     /**
-        Returns a deep copy of the string.
+        Returns a copy of the string.
 
-        That is, the returned string is guaranteed to not share data with this
-        string when using reference-counted wxString implementation.
-
-        This method is primarily useful for passing strings between threads
-        (because wxString is not thread-safe). Unlike creating a copy using
-        @c wxString(c_str()), Clone() handles embedded NULs correctly.
+        This method is obsolete as wxString doesn't use reference-counted
+        implementation any longer and so all string copies are deep and
+        assignment operator or copy constructor can be used instead of this
+        function.
 
         @since 2.9.0
      */