Docstring overhaul

[abelcheung]

Visual Studio Code has support for per-@overload Python docstring a while ago. This feature can serve as special notes for some specific @overload purpose, e.g. function argument usage that can cause problems, like #64.

We can disregard Pycharm, the next most used Python IDE, as far as types-lxml is concerned. As its strict 'everything prefixed with underscore is private' stance directly slaps face on lxml's widespread underscore usage, it becomes a crippled editor for any lxml related projects, such as not showing any superclass method in autocompletion popup for elements.

For all methods, properties and classes not documented in types-lxml, vscode can pick up __doc__ attribute from lxml runtime and display it instead. However, we need not duplicate original lxml docstrings (size is multipled by number of overloads); it is much simpler to point users to API doc on web instead.

Our docstring preferably follows numpy format, which allows more verbose description:

"""One line description of its purpose

Annotation
----------
Notes about specific `@overload`, or provide annotation-only usage for objects.
If no special explanation is necessary, it is possible to omit this section.
Link to wiki whenever necessary.

See Also
--------
[API Documentation](https://lxml.de/apidoc/......)
"""