A plea for more guidance in the darktable manual

[kofa (István Kovács)]

The ‘invalid’ (non-)issue has been discussed several times. It’s not an error indication. Some examples:

• Introducing color calibration module (formerly known as channel mixer rgb) - #142 by aurelienpierre (#142 and #144)
• Experimenting with the new Color Calibration and checker feature - #2 by aurelienpierre
• Color Calibration Area Selection - #3 by priort (#3 and #4)