add cross-ref to rgblevels for 'brightness' (gamma) adjustment in c-b-s

[elstoc]

I'm not sure whether we want to suggest using rgb levels or curves do we? While they're both "rgb" IIRC the controls/interfaces are still display-referred and I think we'd prefer to direct people to scene-referred modules where possible.

[ralfbrown]

True enough, but RGB levels is simpler to use for the cases where the data is within 0...1 (or even a little beyond, since it extrapolates the gamma curve). To properly handle values well above 1, you need to adjust the white and/or gray fulcrum values in colorbalance RGB. Plus levels gives you a picker to set the gamma from a midtone pixel, which colorbalance doesn't.

[elstoc]

Perhaps a rewording then to make it clear that color balance rgb is recommended but that rgb levels provides similar controls to those in the deprecated module.

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