Add a comment explaining allow of wrong_self_convention

Author: darinmorrison

tracing-core/src/span.rs

@@ -68,8 +68,9 @@ impl Id {
         Id(NonZeroU64::new(u).expect("span IDs must be > 0"))
     }
 
-    /// Returns the span's ID as a  `u64`.
+    // Allow `into` by-ref since we don't want to impl Copy for Id
     #[allow(clippy::wrong_self_convention)]
+    /// Returns the span's ID as a  `u64`.
     pub fn into_u64(&self) -> u64 {
         self.0.get()
     }