docs(*): Document cargo features in docs (#2761)

## Description

This adds the features needed for an API to the generated docs for the
remaining workspace crates.  Only the iroh crate itself does not yet
document this.

## Breaking Changes

none

## Notes & open questions

I can for the life of me not get this to work for the iroh crate
itself...  No idea why.  Feel free to give it a try.

## Change checklist

- [x] Self-review.
- [x] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.
- ~~[ ] Tests if relevant.~~
- ~~[ ] All breaking changes documented.~~

Author: flub

iroh-blobs/Cargo.toml

@@ -77,6 +77,7 @@ redb = ["dep:redb"]
 
 [package.metadata.docs.rs]
 all-features = true
+rustdoc-args = ["--cfg", "iroh_docsrs"]
 
 [[example]]
 name = "provide-bytes"

iroh-blobs/src/lib.rs

@@ -25,8 +25,10 @@
 //! [iroh]: https://docs.rs/iroh
 #![deny(missing_docs, rustdoc::broken_intra_doc_links)]
 #![recursion_limit = "256"]
+#![cfg_attr(iroh_docsrs, feature(doc_cfg))]
 
 #[cfg(feature = "downloader")]
+#[cfg_attr(iroh_docsrs, doc(cfg(feature = "downloader")))]
 pub mod downloader;
 pub mod export;
 pub mod format;