Link the new terms, *key* and *index*

Author: encukou

Doc/reference/datamodel.rst

@@ -3234,10 +3234,10 @@ through the object's keys; for sequences, it should iterate through the values.
    via :meth:`!__getitem__`:
 
    - **sequences**, where *subscript* (also called
-     *index*) should be an integer or a :class:`slice` object.
+     :term:`index`) should be an integer or a :class:`slice` object.
      See the :ref:`sequence documentation <datamodel-sequences>` for the expected
      behavior, including handling :class:`slice` objects and negative indices.
-   - **mappings**, where *subscript* is also called the *key*.
+   - **mappings**, where *subscript* is also called the :term:`key`.
      See :ref:`mapping documentation <datamodel-mappings>` for the expected
      behavior.
 

Doc/reference/expressions.rst

@@ -962,7 +962,7 @@ square brackets.
 In the simplest case, the subscript is single expression.
 
 Depending on the type of the object being subscribed, the subscript is
-sometimes called a *key* (for mappings), *index* (for sequences),
+sometimes called a :term:`key` (for mappings), :term:`index` (for sequences),
 or *type argument* (for :term:`generic types <generic type>`).
 Syntactically, these are all equivalent::