Add labels to graphic in blend modes section

[ioalexei]

Description

There is a graphic in the section on blend modes (Fig 8.27) that shows the effect of different blend modes, but the explanation of which blend mode is which is only given in the caption, which is hard to parse at a glance. It would be more helpful if the labels were under each graphic so you don't have to switch focus from graphic to caption and try to count how far left or right each example is.

The use of letters to refer to the caption (as in Fig 8.28 below) is a good alternative if the above would be difficult to make work with translation.

Page URL:
https://docs.qgis.org/3.40/en/docs/user_manual/introduction/general_tools.html#blending-modes