factored iterator adaptor stuff and a few other edits

[SVN r19465]

doc/iterator_adaptor_abstract.rst

@@ -0,0 +1,10 @@
+The ``iterator_adaptor`` is a base class template derived from an
+instantiation of ``iterator_facade``. The core interface functions
+expected by ``iterator_facade`` are implemented in terms of the
+``iterator_adaptor``\ 's ``Base`` template parameter. A class derived
+from ``iterator_adaptor`` typically redefines some of the core
+interface functions to adapt the behavior of the ``Base`` type.
+Whether the derived class models any of the standard iterator concepts
+depends on the operations supported by the ``Base`` type and which
+core interface functions of ``iterator_facade`` are redefined in the
+``Derived`` class.