Add comment about LocalDefId -> DefId

Now there are instructions on how to convert back and forth on both
structs, not just one.

src/librustc_span/def_id.rs

@@ -133,6 +133,8 @@ impl rustc_serialize::UseSpecializedDecodable for DefIndex {}
 
 /// A `DefId` identifies a particular *definition*, by combining a crate
 /// index and a def index.
+///
+/// You can create a `DefId` from a `LocalDefId` using `local_def_id.to_def_id()`.
 #[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Copy)]
 pub struct DefId {
     pub krate: CrateNum,