text: Title: Docs out of sync for `qt_api`
  Body: Hello! Had a test fail with 4.0.0 like:
  
  ```
      @pytest.fixture
      def spawn_terminal(qtbot, blank_pilot_db):
          prefs.clear()
          prefs.set('AGENT', 'TERMINAL')
          pilot_db_fn = blank_pilot_db
          prefs.set('PILOT_DB', pilot_db_fn)
  >      app = qt_api.QApplication.instance()
  
  E       AttributeError: '_QtApi' object has no attribute 'QApplication'
  ```
  
  after looking around closed issues it looks like using `qt_api` directly has largely been deprecated: https://github.com/pytest-dev/pytest-qt/pull/358 and https://github.com/pytest-dev/pytest-qt/issues/365 so I think i'll be able to fix it, but the docs still say to use `qt_api` to test `QApplication` objects this way: 
  
  https://pytest-qt.readthedocs.io/en/latest/qapplication.html?highlight=qt_api#testing-custom-qapplications
  ```
  from pytestqt.qt_compat import qt_api
  
  class CustomQApplication(qt_api.QtWidgets.QApplication):
      def __init__(self, *argv):
          super().__init__(*argv)
          self.custom_attr = "xxx"
  
      def custom_function(self):
          pass
  ```
  
  I realize the reference i'm using: `qt_api.QApplication` is different than `qt_api.QtWidgets.QApplication` but if i'm reading right i should just use `QApplication` itself? or is that a special case vs. the recommendation to do so with `QWidgets` eg here: https://github.com/pytest-dev/pytest-qt/issues/365 ?
  
  seems like a reasonably quick fix to the docs, just lettin ya know :)