MO fallback to old path if some features are not supported yet (#9034)

docs/MO_DG/prepare_model/Model_Optimizer_FAQ.md

@@ -626,4 +626,12 @@ The issue "SyntaxError: 'yield' inside list comprehension" might occur during co
 The following workarounds are suggested to resolve this issue:
 1. Use Python 3.6/3.7 to convert MXNet\* models on Windows
 2. Update MXNet: pip install mxnet=1.7.0.post2
-Note that you might have conflicts between previously installed PyPI dependencies.
+Note that you might have conflicts between previously installed PyPI dependencies.m
+
+#### 105. What does the message "The IR preparation was executed by the legacy MO path. ..." mean? <a name="question-105"></a>
+
+For the models in ONNX* format, there are two available paths of IR conversion. 
+The old one is handled by the old Python* implementation, while the new one uses new C++ frontends. 
+Starting from the 2022.1 version, the default IR conversion path for ONNX models is processed using the new ONNX frontend. 
+Certain features, such as `--extensions` and `--transformations_config`, are not yet supported on the new frontends. 
+The IR conversion falls back to the old path if a user does not select any expected path of conversion explicitly (by `--use_new_frontend` or `--use_legacy_frontend` MO arguments) and unsupported pre-defined scenario is detected on the new frontend path.