The path descriptions in https://nim-lang.org/docs/nimc.html aren't clear, e.g. --cincludes presumably goes at the head of some list, but it's not spelled out, it just says it "modifies" the search path.
There seems to be a convention that PATH is used for filename option arguments, and DIR for dir option arguments, but it's not honored everywhere (e.g. not for the --path option which presumably takes a DIR argument).
It seems like the "Configuration files" section of this page should mention that they take command line args per line as their format. I remembered this from somewhere. It's also pretty unexpected that the config files require the option arguments to be quoted (e.g. --cincludes="/foo/bar") when they don't require that when used on the command line, so this should be mentioned in this section as well.