message: Pass --crate-type to rustdoc
  This supports the corresponding rustc PR. To enable rustdoc to properly
  document macros, we mirror the '--crate-type' flag used by rustc.
  Currently, all crate types other than 'proc-macro' are ignored by
  rustdoc.
diff: @@ -140,6 +140,10 @@ impl<'cfg> Compilation<'cfg> {
  if target.edition() != Edition::Edition2015 {
  p.arg(format!("--edition={}", target.edition()));
  }
  + for crate_type in target.rustc_crate_types() {
  + p.arg("--crate-type").arg(crate_type);
  + }
  +
  Ok(p)
  }
  