[MLIR][ODS] Improve diagnostic for using a property as an anchor

The diagnostic make it more clear that it does not have an optionalParser
field defined.

Fix #147020

Author: joker-eph

mlir/tools/mlir-tblgen/OpFormatGen.cpp

@@ -3779,7 +3779,8 @@ LogicalResult OpFormatParser::verifyOptionalGroupElement(SMLoc loc,
         Property prop = propEle->getVar()->prop;
         if (isAnchor && !(prop.hasDefaultValue() && prop.hasOptionalParser()))
           return emitError(loc, "only properties with default values "
-                                "that can be optionally parsed "
+                                "that can be optionally parsed (have the `let "
+                                "optionalParser = ...` field defined) "
                                 "can be used to anchor an optional group");
         return success();
       })