Add information about negative indexes to sequence datamodel doc (pyt…

…hon#110903)

Co-authored by Terry Jan Reedy

Doc/reference/datamodel.rst

@@ -299,14 +299,17 @@ Sequences
 These represent finite ordered sets indexed by non-negative numbers. The
 built-in function :func:`len` returns the number of items of a sequence. When
 the length of a sequence is *n*, the index set contains the numbers 0, 1,
-..., *n*-1.  Item *i* of sequence *a* is selected by ``a[i]``.
+..., *n*-1.  Item *i* of sequence *a* is selected by ``a[i]``. Some sequences,
+including built-in sequences, interpret negative subscripts by adding the
+sequence length. For example, ``a[-2]`` equals ``a[n-2]``, the second to last
+item of sequence a with length ``n``.
 
 .. index:: single: slicing
 
 Sequences also support slicing: ``a[i:j]`` selects all items with index *k* such
 that *i* ``<=`` *k* ``<`` *j*.  When used as an expression, a slice is a
-sequence of the same type.  This implies that the index set is renumbered so
-that it starts at 0.
+sequence of the same type. The comment above about negative indexes also applies
+to negative slice positions.
 
 Some sequences also support "extended slicing" with a third "step" parameter:
 ``a[i:j:k]`` selects all items of *a* with index *x* where ``x = i + n*k``, *n*