Improve Documentation for QMatrix4x4

There have been a number of reports of confusion about how functions in QMatrix4x4 operate. QTBUG-18096 relates to the translate function which Multiplies this matrix by another that translates coordinates by the components of vector.

Since matrix multiplication is sensitive to ordering it should at least state whether this is pre-multiplying or post-multiplying. It does in fact post-multiply, which is the opposite of what you normally do with QMatrix4x4 when applying a transform to a QVector3D.

Also since the QTransform class inter-operates with this class, and has some similar functions, users might wrongly assume that the ordering would be similar when in fact it is the opposite. The class documentation for QTransform features diagrams and extensive explanation where the class documentation for this class is one line.

Lastly there seems to be no specific examples that are linked to the documentation although the class is widely used. Snippets would be a great help in understanding how to use it.