docs(iroh-net): Document cargo features needed for APIs (#2759)

## Description

This uses the feature of docs.rs to enable showing which cargo
features need to be enabled for APIs.  This is much friendlier for
users.

## Breaking Changes

None

## Notes & open questions

I'm trying to do this for the entire workspace but for some reason am
not getting this to work on other crates yet.  This is all such a
mystery and mess.  Just get this in stable rust already and do this by
default without needing extra annotations.

I'll try and do the rest of the workspace still in a followup PR, but
at least this might make the release so we get to see if it works on
actual docs.rs as well.

## 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

Cargo.toml

@@ -34,5 +34,15 @@ incremental = false
 [workspace.lints.rust]
 missing_debug_implementations = "warn"
 
+# We use this --cfg for documenting the cargo features on which an API
+# is available.  To preview this locally use: RUSTFLAGS="--cfg
+# iroh_docsrs cargo +nightly doc --all-features".  We use our own
+# iroh_docsrs instead of the common docsrs to avoid also enabling this
+# feature in any dependencies, because some indirect dependencies
+# require a feature enabled when using `--cfg docsrs` which we can not
+# do.  To enable for a crate set `#![cfg_attr(iroh_docsrs,
+# feature(doc_cfg))]` in the crate.
+unexpected_cfgs = { level = "warn", check-cfg = ["cfg(iroh_docsrs)"] }
+
 [workspace.lints.clippy]
 unused-async = "warn"