factoring the specialized iterator adaptors

[SVN r19467]

doc/function_output_iterator_abstract.rst

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+The function output iterator adaptor makes it easier to create custom
+output iterators. The adaptor takes a unary function and creates a
+model of Output Iterator. Each item assigned to the output iterator is
+passed as an argument to the unary function.  The motivation for this
+iterator is that creating a conforming output iterator is non-trivial,
+particularly because the proper implementation usually requires a
+proxy object.
+