Doc: Do not advise to overwrite QWidget::contextMenuEvent()

There's also QWidget::setContextMenuPolicy and
QWidget::customContextMenuRequested, which is arguably a better
approach. Anyhow, this isn't specific to QWebEngineView, so
let's leave it at that. Finally, there's no need to call
triggerPageAction if you add an action to a menu.

Change-Id: I0c84038746a78316ed6b90a98b68fa5eb95fbad5
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>