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