[mlir] Update comment about propertiesAttr (NFC)

The comment is misleading because `propertiesAttr` is not actually
ignored when the operation isn't unregistered.

stack-info: PR: #89631, branch: users/Mogball/stack/1

Author: Mogball

mlir/include/mlir/IR/OperationSupport.h

@@ -960,9 +960,12 @@ struct OperationState {
   /// Regions that the op will hold.
   SmallVector<std::unique_ptr<Region>, 1> regions;
 
-  // If we're creating an unregistered operation, this Attribute is used to
-  // build the properties. Otherwise it is ignored. For registered operations
-  // see the `getOrAddProperties` method.
+  // This Attribute is used to opaquely construct the properties of the
+  // operation. If we're creating an unregistered operation, the Attribute is
+  // used as-is as the Properties storage of the operation. Otherwise, the
+  // operation properties are constructed opaquely using its
+  // `setPropertiesFromAttr` hook. Note that `getOrAddProperties` is the
+  // preferred method to construct properties from C++.
   Attribute propertiesAttr;
 
 private: