Add documentation for handling optional arguments.

src/dialect/onnx/onnx.td

@@ -78,6 +78,17 @@ def ONNXEntryPointOp: ONNX_Op<"EntryPoint"> {
   }];
 }
 
+//===----------------------------------------------------------------------===//
+// ONNX Operations for handling optional arguments
+//===----------------------------------------------------------------------===//
+
+// To allow pattern matching on operations with optional arguments/outputs we
+// implement variants of the original ONNX dialect operations. The ONNX
+// operations automatically generated by the `gen_doc.py` script and included
+// in the `onnxop.inc` file have all optional arguments and outputs present.
+// In the operations below we include the variants with missing operands
+// or outputs. This decision affects only ONNX operations with optional
+// arguments not ONNX operations with variadic operands.
 
 def ONNXFullGemmOp: ONNX_Op<"FullGemm",
     [NoSideEffect, DeclareOpInterfaceMethods<ShapeInferenceOpInterface>]> {