factoring the specialized iterator adaptors

[SVN r19467]

doc/permutation_iterator_body.rst

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+The adaptor takes two arguments:
+
+  * an iterator to the range V on which the permutation
+    will be applied
+  * the reindexing scheme that defines how the
+    elements of V will be permuted.
+
+Note that the permutation iterator is not limited to strict
+permutations of the given range V.  The distance between begin and end
+of the reindexing iterators is allowed to be smaller compared to the
+size of the range V, in which case the permutation iterator only
+provides a permutation of a subrange of V.  The indexes neither need
+to be unique. In this same context, it must be noted that the past the
+end permutation iterator is completely defined by means of the
+past-the-end iterator to the indices.