Clarified AreaSeries documentation...
Clarified AreaSeries documentation
Added note about using a line series only for area series. If the same
line series is added to chart and area series it will cause problems.
Change-Id: Ie9fbc1f0741e139d82bd3d705b17b0369afc3dc9
Task-number: QTRD-3516
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@theqtcompany.com>