It treats something like feature=bar as a single MetaWord instead of a MetaNameValue. This is due to the code on lines 70-73 of librustdoc/test.rs:

cfg.extend(cfgs.into_iter().map(|cfg_| {
    let cfg_ = token::intern_and_get_ident(&cfg_);
    P(dummy_spanned(ast::MetaWord(cfg_)))
}));

I have a PR for this that I will open shortly.