handle changes in nullability

src/change.rs

@@ -55,10 +55,31 @@ pub enum ChangeComparison {
 
 #[derive(Debug)]
 pub enum ChangeClass {
+    /// This change is backward incompatible. A client on the old version may
+    /// not be able to communicate with a server on the new version.
+    ///
+    /// This represents an input type becoming more restricted (so an old client
+    /// can send request data that a new server will reject), or an output type
+    /// becoming less restricted (so a new server can send response data that an
+    /// old client will reject).
     BackwardIncompatible,
+    /// This change is forward incompatible. A server on the old version may
+    /// not be able to communicate with a client on the new version.
+    ///
+    /// This represents an input type becoming less restricted (so a new client
+    /// can send request data that an old server will reject), or an output type
+    /// becoming more restricted (so an old server can send response data that a
+    /// new client will reject).
     ForwardIncompatible,
+    /// This change is bidirectionally incompatible. A server on the old version
+    /// may not be able to communicate with a client on the new version, and
+    /// vice versa.
     Incompatible,
+    /// This change is trivial (e.g., metadata changed). Servers and clients
+    /// across versions will not be affected.
     Trivial,
+    /// This change is not handled by `drift` yet, and it should be assumed to
+    /// be bidirectionally incompatible.
     Unhandled,
 }