Slight cleanup and added some documentation.

[UebelAndre]

Nothing crazy, just noticed a missing doc attr from #525 and wanted to add it.

And FYI. Lists will print with proper formatting.

print("{} expected a value in `{}` but got `{}`".format(
    ctx.label,
    _error_format_values,
    raw,
))

DEBUG: /Users/user/Code/rules_rust/rust/private/rustc.bzl:862:10: //:error_format expected a value in `["human", "json", "short"]` but got `human`