[Core] Document all exported core methods with type hints

Standardize docstrings in core.py to google standard.
Remove type hints in docstrings in favor of the ones in method signatures.

Use function signature type hints in autodoc generated docs.

Change Deferred type hints to strings.

Older versions of twisted (<21.7) don't
support generic Deferred type hinting,
this prevents crashes on those versions.

Closes: #359