[doc_auto_cfg] feature gated macros are not marked with feature badge

I have some macros behind a feature flag and when generating the doc, they don't have the feature badge.
I had to mark each of them manually with `doc(cfg(feature = "my_feature"))` (and in an `cfg_attr` to keep compiling on stable)

I'm not sure if this is intended or not but I couldn't find anything on the subject on this repo :)