`#[doc(cfg(..)]` does not respect `check-cfg`

[clubby789]

Original context

check_matches emits a buffered lint, but rustdoc never emits this, so unknown cfgs are silently expected.

[camelid]

cc @GuillaumeGomez

[GuillaumeGomez]

I don't even understand the problem here. I ran locally with:

#![feature(doc_cfg)]

#[cfg(feature = "unk")]
#[doc(cfg(feature = "unk"))]
pub fn f() {}

I added a print after cfg_matches of eprintln!("++++> {:?}", tcx.sess.psess.buffered_lints); and it displays Lock { data: [] }. So I suppose rustc is missing some information to be able to generate warning about this, but which one?