Improve documentation for *XYModelMapper QML Type

Based on my experience, the documentation for HXYModelMapper and VXYModelMapper QML Types is not so complete and clear.

Moreover no official examples are available, and so it's almost impossible to understnad how to use them.

I suggest to improve the documentation and to provide official usage examples.

As an example, in the Detailed description I would modify in the following way:

A vertical model mapper is used to create a connection between a line, spline, or scatter series and the data model (TableModel is the natural choice) that has a X and a Y column and holds the data point coordinate for the XYSeries in rows.

If I'm right the HXY and VHY Detailed Description sections are swapped (VXY holds 2 columns and one row for every point in the series).