new files for iterator concepts

[SVN r22004]

doc/LvalueIterator.rst

@@ -0,0 +1,21 @@
+Lvalue Iterator Concept
+.......................
+
+The *Lvalue Iterator* concept adds the requirement that the return
+type of ``operator*`` type be a reference to the value type of the
+iterator.
+
++-------------------------------------------------------------+
+| Lvalue Iterator Requirements                                |
++-------------+-----------+-----------------------------------+
+|Expression   |Return Type|Note/Assertion                     |
++=============+===========+===================================+
+|``*a``       | ``T&``    |``T`` is *cv*                      |
+|             |           |``iterator_traits<X>::value_type`` |
+|             |           |where *cv* is an optional          |
+|             |           |cv-qualification.                  |
+|             |           |pre: ``a`` is                      |
+|             |           |dereferenceable. If ``a            |
+|             |           |== b`` then ``*a`` is              |
+|             |           |equivalent to ``*b``.              |
++-------------+-----------+-----------------------------------+