docs: add missing backticks in documentation

this improve readability of documentation.
enable associated clippy lint `doc_markdown`

src/util/background/worker.rs

@@ -34,11 +34,11 @@ pub trait Worker: Send {
 	}
 
 	/// Work: do a basic unit of work, if one is available (otherwise, should return
-	/// WorkerState::Idle immediately).  We will do our best to not interrupt this future in the
+	/// `WorkerState::Idle` immediately).  We will do our best to not interrupt this future in the
 	/// middle of processing, it will only be interrupted at the last minute when Garage is trying
 	/// to exit and this hasn't returned yet. This function may return an error to indicate that
 	/// its unit of work could not be processed due to an error: the error will be logged and
-	/// .work() will be called again after a short delay.
+	/// .`work()` will be called again after a short delay.
 	async fn work(&mut self, must_exit: &mut watch::Receiver<bool>) -> Result<WorkerState, Error>;
 
 	/// Wait for work: await for some task to become available.  This future can be interrupted in