Update Histogram Dialog section
- Document "Luminance" channel dropdown value added in gimp@f4cb2dd8
- Document Linear / Non-Linear / Perceptual buttons (TRC) added in gimp@e09e563a
- Update screenshots (three additional TRC buttons displayed)
- Remove trivial outdated histogram-drop-down-menu.png screenshot of channel dropdown menu
- Remove histogram-log.png screenshot as text below is sufficiently self-explanatory