autoreload: add some notes about systemfd/listenfd (#696)

Author: JohnTitor

docs/autoreload.md

@@ -20,6 +20,15 @@ watchexec -w src -r cargo run
 
 ## Historical Note
 
-An old version of this page recommended using a combination of systemfd and listenfd, but this has many gotchas and was difficult to integrate properly, especially when part of a broader development workflow. We consider [`watchexec`] to be sufficient for auto-reloading purposes.
+An old version of this page recommended using a combination of [`systemfd`] and [`listenfd`], but this has many gotchas and was difficult to integrate properly, especially when part of a broader development workflow. We consider [`watchexec`] to be sufficient for auto-reloading purposes.
+
+If you still want to use [`systemfd`]/[`listenfd`], keep these common gotchas in mind:
+
+- It requires code changes to accept an externally provided listener, so you need extra setup (and often an extra dependency) in your server code instead of a simple `bind`.
+- When a watcher process sits between `systemfd` and your server (for example, `cargo watch` or `watchexec`), you must disable the `LISTEN_PID` check (for example, `systemfd --no-pid`). Otherwise `listenfd` ignores the passed socket and your app falls back to binding a port that is already in use.
+- systemd socket activation itself is Linux-only. [`systemfd`] emulates socket passing on macOS/Linux and uses a custom protocol on Windows, so behavior differs across platforms.
+- Because [`systemfd`] keeps the listening socket open while a rebuild happens, the port can look "open" even when your app is not running yet; requests may hang during slow or failed rebuilds.
 
 [`watchexec`]: https://github.com/watchexec/watchexec
+[`listenfd`]: https://github.com/mitsuhiko/listenfd
+[`systemfd`]: https://github.com/mitsuhiko/systemfd