Doc: note that QImage::transformed() may return different format

The user wants transform the QImage.

The expected behaviour:

QImage transformed and keeps its format (if the QImage had a RGB32 format it must have after the transformation done the RGB32 format)

What actually happens:

After the transformation the QImage format is changed from RGB32 to ARGB32.

Steps to reproduce:

1. Download and unpack the application attached.
2. Run it as it is and see the application output.
3. Comment out t.rotate(1); in main.cpp, re-compile application and see the output