Rollup merge of #45090 - rust-lang:petrochenkov-patch-1, r=alexcrichton

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

Are there any plans to use an external archiver in the future?
IIRC, it was used before, but its use was replaced with LLVM's built-in archive management machinery. I can't found a relevant PR though. EDIT: Found it - https://github.com/rust-lang/rust/pull/26926!

The `-C` option is stable so it still can't be removed right away even if there are no plans to use it (but maybe it can be deprecated?).
Target specifications have a field for archiver as well, which is unused too (these ones are unstable, so I guess it can be removed).

r? @alexcrichton

src/librustc/session/config.rs

@@ -864,7 +864,8 @@ fn parse_optimization_fuel(slot: &mut Option<(String, u64)>, v: Option<&str>) ->
          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],