Documentation: Update list of roles and their descriptions

[carolalynn]

https://opencolorio.readthedocs.io/en/latest/guides/authoring/config_syntax.html?highlight=roles#roles

https://opencolorio.readthedocs.io/en/latest/configurations/aces_1.0.3.html#id4

[chrisbrejon]

This might overlap with the ACES documentation but I would love to see as well an explanation for each role, as why a color space or an other has been chosen. For example, "Output - sRGB" for the color_picking role. I know lots of artists have scratched their heads on this one. ;-)

[zachlewis]

Perhaps it's worth documenting (and adding Enums) for rendering, compositing_linear, aces_interchange, and cie_xyz_d65_interchange?

Regarding the reference role:

Although it's quite clear in the docs now, historically, there's been ambiguity regarding what "reference" means. When I first encountered it, the role was defined in the docs as "Colorspace used for reference imagery (e.g sRGB images from the internet)". This ambiguity led the ACES-1+ OCIO authors to select a data colorspace for the reference role "as a pass on making a decision" (see https://community.acescentral.com/t/why-utility-raw-as-reference-for-aces1-0-3-and-not-aces2065-1/1378/11); consequently, I quite often (perhaps usually) see TD-authored / edited configs pointing reference to a data space as well.

Three questions:

1. Is any of this worth bringing up in the documentation? I feel like DCC developers who had previously implemented OCIO, or old-school users of OCIO might find the information worthwhile; but everyone else would find it unnecessary, pedantic, and/or confusing.
2. Is anyone aware of DCC applications or tools that require or expect reference to point to a "reference photography" color space?
3. Is anyone aware of DCC applications or tools that behave differently when the reference role points to a data colorspace instead of the reference [scene-linear "connector"] color space?

Anecdotally, I've been told that switching the reference role from a data color space to the reference color space affects the behavior of the color picker in Mari (2?), but I have not confirmed this for myself.