warning: unexpected `cfg` condition name: `foo`
  --> $DIR/doc-cfg-check-cfg.rs:13:11
   |
LL | #[doc(cfg(foo))]
   |           ^^^
   |
   = help: to expect this configuration use `--check-cfg=cfg(foo)`
   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
   = note: `#[warn(unexpected_cfgs)]` on by default

warning: 1 warning emitted