QtHelp: Document that registered help files share one origin

All registered documentation is addressed through the single qthelp
URL scheme, so a renderer that follows web security rules might treat
all of it as one origin. The Security Considerations section didn't
say that registering one untrusted help file therefore exposes every
other registered help file, and the application interfaces available
to the rendered pages.

Document this, recommend per-namespace origins for viewers built on a
web engine, and document that QHelpEngineCore::fileData() returns
untrusted, unfiltered content.

Fixes: QTBUG-150685
Pick-to: 6.12 6.8
Change-Id: Ie1162adfec2b790828e4b1e90ea8be26062b9b46
Reviewed-by: Kai Köhne <kai.koehne@qt.io>