I think I gave the impression in an earlier comment that "we shouldn't add comments for things that are self-explanatory", which either is not what I meant or is not how I feel *now* after thinking about it.
I think there shouldn't be an expectation that we *need* to document every field just for the sake of documenting every field, but if you feel like a field would be improved with documentation, I don't think that there's any reason to bikeshed that. (Not least because, as the most recent person to work on the HLSL compiler, your thoughts should be given the heaviest weight.) Similarly explaining what a thing is doesn't necessarily hurt either. Especially for something like hlsl_deref which is kind of weird and non-obvious. (Similarly I could see the difference between hlsl_ir_function and hlsl_ir_function_decl being confusing.)