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.
diff --git a/Doc/c-api/object.rst b/Doc/c-api/object.rst
index d895547..a47183c 100644
--- a/Doc/c-api/object.rst
+++ b/Doc/c-api/object.rst
@@ -160,11 +160,11 @@
    a string similar to that returned by :c:func:`PyObject_Repr` in Python 2.
    Called by the :func:`ascii` built-in function.
 
+   .. index:: string; PyObject_Str (C function)
+
 
 .. c:function:: PyObject* PyObject_Str(PyObject *o)
 
-   .. index:: builtin: str
-
    Compute a string representation of object *o*.  Returns the string
    representation on success, *NULL* on failure.  This is the equivalent of the
    Python expression ``str(o)``.  Called by the :func:`str` built-in function