Clarify where support for negative indices fall in the language hierarchy.

Doc/reference/expressions.rst

@@ -518,11 +518,18 @@ whose value is one of the keys of the mapping, and the subscription selects the
 value in the mapping that corresponds to that key.  (The expression list is a
 tuple except if it has exactly one item.)
 
-If the primary is a sequence, the expression (list) must evaluate to an integer.
-If this value is negative, the length of the sequence is added to it (so that,
-e.g., ``x[-1]`` selects the last item of ``x``.)  The resulting value must be a
-nonnegative integer less than the number of items in the sequence, and the
-subscription selects the item whose index is that value (counting from zero).
+If the primary is a sequence, the expression (list) must evaluate to an integer
+or a slice (as discussed in the following section).
+
+The formal syntax makes no special provision for negative indices in
+sequences; however, built-in sequences all provide a :meth:`__getitem__`
+method that interprets negative indices by adding the length of the sequence
+to the index (so that ``x[-1]`` selects the last item of ``x``).  The
+resulting value must be a nonnegative integer less than the number of items in
+the sequence, and the subscription selects the item whose index is that value
+(counting from zero). Since the support for negative indices and slicing
+occurs in the object's :meth:`__getitem__` method, subclasses overriding
+this method will need to explicitly add that support.
 
 .. index::
    single: character