Add doc comments for `ItemReferenceArgs`

Enhancement Request

Explanation

Even though each argument usable in ItemReferenceArg has a description on the wiki:

https://dwarffortresswiki.org/index.php/DF2014:Item_token

None of these are present in the actual code as doc comments; they should be added for later "hover text description" purposes.

However, the way to do this is not clear because code implementation for it doesn't actually have an enum value for each possible token value (unlike MaterialTokenArg), so that will need to be figured out; either way, somehow this information needs to be present.