Once stabilised, use the doc_cfg feature to automatically tag items with their required features in documentation

[mitchmindtree]

See here for info about the attribute.

I tried to get this going as a part of #120, however it seems that the attribute is ignored within crate dependencies. That is, when documenting dasp, the re-exported dependencies did not provide any information about their own features, only providing tags for the dasp-level features.

There's an issue following development of the feature here rust-lang/rust#43781.