Add iterator test notes.

[SVN r21019]

status/explicit-failures-markup.xml

@@ -29,4 +29,28 @@
     </note>
   </mark>
 
+  <mark library="iterator" test="interoperable_fail" toolset="*">
+    <note author="D. Abrahams">
+      This failure is caused by a compiler bug.  Templated operators
+      which combine different iterators built with iterator_facade or
+      iterator_adaptor be present in an overload set even when those
+      iterators are not interoperable.  The usual result is that error
+      messages generated by illegal use of these operators will be of
+      lower quality.
+    </note>
+  </mark>
+
+  <mark library="iterator" test="is_convertible_fail" toolset="*">
+    <note author="D. Abrahams">
+      This failure is caused by a compiler bug.
+      <code>is_convertible&lt;T,U&gt;::value</code> may be true for unrelated
+      iterators <code>T</code> and <code>U</code>
+      (including many of the Boost specialized adaptors) which use
+      <code>enable_if_convertible</code> to restrict the applicability
+      of converting constructors, even when <code>T</code> is not
+      convertible to <code>U</code> because instantiating the
+      conversion will cause a compilation failure.
+    </note>
+  </mark>
+
 </explicit-failures-markup>