Minor fixes to C API docs (GH-31501)

* C API docs: move PyErr_SetImportErrorSubclass docs

It was in the section about warnings, but it makes more sense to
put it with PyErr_SetImportError.

* C API docs: document closeit argument to PyRun_AnyFileExFlags

It was already documented for PyRun_SimpleFileExFlags.

* textual fixes to unicode docs

* Move paragraph about tp_dealloc into tp_dealloc section

* __aiter__ returns an async iterator, not an awaitable

Doc/c-api/exceptions.rst

@@ -253,6 +253,14 @@ For convenience, some of these functions will always return a
    .. versionadded:: 3.3
 
 
+.. c:function:: PyObject* PyErr_SetImportErrorSubclass(PyObject *exception, PyObject *msg, PyObject *name, PyObject *path)
+
+   Much like :c:func:`PyErr_SetImportError` but this function allows for
+   specifying a subclass of :exc:`ImportError` to raise.
+
+   .. versionadded:: 3.6
+
+
 .. c:function:: void PyErr_SyntaxLocationObject(PyObject *filename, int lineno, int col_offset)
 
    Set file, line, and offset information for the current exception.  If the
@@ -320,13 +328,6 @@ an error value).
    :mod:`warnings` module and the :option:`-W` option in the command line
    documentation.  There is no C API for warning control.
 
-.. c:function:: PyObject* PyErr_SetImportErrorSubclass(PyObject *exception, PyObject *msg, PyObject *name, PyObject *path)
-
-   Much like :c:func:`PyErr_SetImportError` but this function allows for
-   specifying a subclass of :exc:`ImportError` to raise.
-
-   .. versionadded:: 3.6
-
 
 .. c:function:: int PyErr_WarnExplicitObject(PyObject *category, PyObject *message, PyObject *filename, int lineno, PyObject *module, PyObject *registry)