Doc: Fix QDoc warnings for Qt Charts signals

Mark several public-API signals \internal since Qt Charts is in
maintenance mode and these APIs were never published. The signals
remain in the public ABI; \internal only hides them from generated
documentation.

For QXYSeries::selectedColor and QLegend::markerShape), add real \fn
documentation instead of suppressing them.

Add missing parameter names to the QBarSet::borderColorChanged and
labelColorChanged \fn blocks so QDoc can resolve \a color.

Task-number: QTBUG-146792
Change-Id: I5bfa783e4006cd83f7c9e6fab28ddb9ce17f142d
Pick-to: 6.11
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: Topi Reinio <topi.reinio@qt.io>