Improve the comment on DataObjectOps::save (#350)

Relevant to #349

Author: Electron100

butane_core/src/lib.rs

@@ -156,7 +156,12 @@ pub trait DataObjectOps<T: DataObject> {
             .nth(0))
     }
 
-    /// Save the object to the database.
+    /// Save the object to the database, handling both inserts and updates.
+    ///
+    /// If the object has an AutoPk that is uninitialized, save will always
+    /// perform an insert. If the AutoPk is initialized or there is no AutoPk,
+    /// save will perform an upsert (insert or replace).
+    /// After saving the main object, many-to-many relationships it holds are also saved.
     async fn save(&mut self, conn: &impl ConnectionMethods) -> Result<()>
     where
         Self: DataObject,