Update Context-related docs

[dbr]

From the nuke-users mailing list, "[Nuke-users] OCIO frustrations" thread:

One of the things that OCIO seems to mention frequently in the documentation is its ability to recognize context/environment variables. So, you can look for ${SEQ}/${SHOT}/grade/${SHOT}.cdl and apply it, in theory, as part of an OCIO config.

The only problem is, it would appear that OCIO only calculates these variables ONCE at application startup? I have scripts that run when the onScriptLoad event handler is triggered that alter these environment variables based on the script that is opened. However, it would appear that OCIO is not listening for any changes to the environment variables while Nuke is running?

This is a pretty reasonable thing to be confused by, and the context override tab should be mentioned (in a Nuke specific part of the docs?)

Unrelatedly, http://opencolorio.org/userguide/contexts.html#context-per-shot-grade should be updated to use a Look instead of the lin_graded colorspace