Add a str class entry to the "Text Sequence Type" section (issue #16209).

This commit also moves the documentation for the str built-in function to
the new class entry.  Links to :class:`str` now go to the class entry with
the string methods immediately afterwards.

Doc/extending/newtypes.rst

@@ -982,13 +982,13 @@ done.  This can be done using the :c:func:`PyErr_Fetch` and
    }
 
 
+.. index::
+   single: string; object representation
+   builtin: repr
+
 Object Presentation
 -------------------
 
-.. index::
-   builtin: repr
-   builtin: str
-
 In Python, there are two ways to generate a textual representation of an object:
 the :func:`repr` function, and the :func:`str` function.  (The :func:`print`
 function just calls :func:`str`.)  These handlers are both optional.