[formrecognizer] Review feedback

[catalinaperalta]

• make sure samples that check field_elements include an else for elements that dont have a kind: https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/formrecognizer/azure-ai-formrecognizer/samples/sample_get_bounding_boxes.py (context: this doesn't make sense / the code will never be reached. FormElement is a base model we created in the SDK out of convenience for all "element" type models to inherit from, it doesn't exist in the REST API. We parse the json pointer to return a list of field elements which are words, lines, and selection marks. If a json pointer is returned with a bad path, we raise an exception b/c the service is returning bad pointers.)
• check docstrings that appear strange in the APIView
• check if the change in the API version enum for 2.0 is a breaking change? [formrecognizer] regenerates on v2.1 #18551
• update ivars for TextAppearance and TextStyle [formrecognizer] flatten style properties on text appearance model #18562
• Remove polling interval from docstrings
• add FormReadingOrder enum?