docs(ir): clarify localness of socketidx

crates/ir/src/id.rs

@@ -58,6 +58,13 @@ pub struct Socket {
     pub idx: SocketIdx,
 }
 
-/// Where a [`Socket`] is on an **instruction**.
+/// Where a [`Socket`] is on one **instruction**.
+///
+/// Note that this does **not** identify a [`Socket`] globally.
+/// There may be multiple [`Socket`]s sharing the same [`SocketIdx`],
+/// but on different [`Instruction`]s.
+///
+/// This really only serves for denoting where a socket is,
+/// when it's already clear which instruction is referred to.
 #[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize, Deserialize)]
 pub struct SocketIdx(pub u16);