Docs: Rewrite system cursor

[KeyWeeUsr]

Add more info about fallbacks to other cursors according to the used SDL2 files on specific platforms + a note about only being available on desktops. For more info see files at:

https://hg.libsdl.org/SDL/file/<hash>/src/video/<provider>/<mouse .c/.m/.cpp file>

Feel free to comment about specific changes. If the cursor is different as stated in the docs, comment with a screenshot included. You can run this example:

from kivy.clock import Clock
from kivy.lang import Builder
from kivy.app import runTouchApp
from kivy.uix.button import Button
from kivy.core.window import Window as win
from kivy.uix.gridlayout import GridLayout


class Root(GridLayout):
    def __init__(self, **kwargs):
        super(Root, self).__init__(**kwargs)
        cursors = (
            'arrow', 'ibeam', 'wait', 'crosshair', 'wait_arrow',
            'size_nwse', 'size_nesw', 'size_we', 'size_ns',
            'size_all', 'no', 'hand'
        )

        for i in cursors:
            button = Button(text=i)
            button.bind(on_release=lambda ins: self.reset(ins.text))
            self.add_widget(button)

    def reset(self, cursor):
        win.set_system_cursor(cursor)
        Clock.schedule_once(
            lambda *dt: win.set_system_cursor('arrow'), 1.0
        )


runTouchApp(Root(cols=4))