QDom: give name to all anonymous parameters as well as too generic ones

This patch has several different goals:
- Make the QDom classes easier to use through PySide6 by avoiding the
  the generation of variable names that have no meaning.
- Make the code cleaner as using unrelated single letter parameter is
  not the best way to convey the information.
- Improve the documentation.
- Make qdoc's work easier

Fixes: QTBUG-121670
Task-number: QTBUG-121468
Change-Id: Ia31d119b3a1cc3c92ed6f77a60280191db3b74bc
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

Author: sgaist