Clarify documentation of rgbgfx -C/--color-curve

[Rangi42]

Fixes #1863

[Rangi42]

@quinnyo Do you think the above rewrite is more clear?

[quinnyo]

Thanks for your efforts!

This is certainly an improvement, but it seems to be a better description of what I understood/expected the feature to be. It doesn't clarify the behaviour that caught me out.

The observed behaviour (#1863) indicates that the transformation is not applied when outputting palettes, but more universally to all colour inputs. The behaviour that surprised me is that this includes colours from a palette in the native format of the target platform.
Given a palette in the native format of the target platform, it would not be unreasonable to assume the palette is in the native colour space of the target platform.

Additionally, the phrase "when outputting palettes" may imply that the behaviour is conditional on whether or not a palette file is being written out.