Close #18990: remove root attribute from XMLPullParser

- this was an internal implementation detail for iterparse
- this has been changed to use a new private method instead
- XMLPullParser.close docs are now more explicit about not
  returning a root element and instead direct users towards
  read_events
- also added missing docstrings and clarified some details
  related to exactly *when* events are consumed from the
  internal queue

(Initial patch by Stefan Behnel)