Document that -C ar=PATH doesn't do anything

Author: petrochenkov

src/librustc/session/config.rs

@@ -874,7 +874,8 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
          build_codegen_options, "C", "codegen",
          CG_OPTIONS, cg_type_desc, cgsetters,
     ar: Option<String> = (None, parse_opt_string, [UNTRACKED],
-        "tool to assemble archives with"),
+        "tool to assemble archives with (has no effect currently, \
+         rustc doesn't use an external archiver)"),
     linker: Option<String> = (None, parse_opt_string, [UNTRACKED],
         "system linker to link outputs with"),
     link_arg: Vec<String> = (vec![], parse_string_push, [UNTRACKED],