update documentation about HTML5 drag of capybara driver

[YusukeIwaki]

We can handle HTML5 drag/drop with Playwright 1.12.0 or later. (Special thanks to microsoft/playwright#6207)

  it 'drag and drop' do
    Capybara.app_host = 'https://www.w3schools.com'
    visit '/html/html5_draganddrop.asp'
    find('#drag1').drag_to(find('#div2'))
  end

This example works as expected with latest Chrome.

However when we execute https://github.com/teamcapybara/capybara/blob/master/lib/capybara/spec/session/node_spec.rb, it doesn't works as Capybara expects...

    #drag_to
      HTML5
        should HTML5 drag and drop an object
        should HTML5 drag and drop an object child
        should set clientX/Y in dragover events (FAILED - 1)
        should preserve clientX/Y from last dragover event (FAILED - 2)
        should not HTML5 drag and drop on a non HTML5 drop element
        should HTML5 drag and drop when scrolling needed (FAILED - 3)
        should drag HTML5 default draggable elements (FAILED - 4)
        should work with SortableJS (FAILED - 5)
        should drag HTML5 default draggable element child (FAILED - 6)
        should simulate a single held down modifier key
        should simulate multiple held down modifier keys
        should support key aliases

For example anchor links are draggable by default, but not draggable with Playwright.

This is difficult to explain for users. So just describe "not fully supported"

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