doc: adjust IpfsOptions::span

Author: Joonas Koivunen

src/lib.rs

@@ -142,8 +142,11 @@ pub struct IpfsOptions {
     /// Bound listening addresses; by default the node will not listen on any address.
     pub listening_addrs: Vec<Multiaddr>,
 
-    /// The span for tracing purposes. All futures returned by `Ipfs`, background task actions and
-    /// swarm actions are instrumented with this span or spans referring to this as their parent.
+    /// The span for tracing purposes, `None` value is converted to `tracing::trace_span!("ipfs")`.
+    ///
+    /// All futures returned by `Ipfs`, background task actions and swarm actions are instrumented
+    /// with this span or spans referring to this as their parent. Setting this other than `None`
+    /// default is useful when running multiple nodes.
     pub span: Option<Span>,
 }