[textanalytics] add :keyword: to docstrings for TextDocumentInput and DetectLanguageInput

[kristapratico]

These are models that a user can instantiate and pass as input into a client method. We should additionally include the keyword arguments possible in the docstrings for clearer documentation.

E.g.
:keyword str id: A unique, non-empty document identifier.

https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/textanalytics/azure-ai-textanalytics/azure/ai/textanalytics/_models.py#L743-L751

https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/textanalytics/azure-ai-textanalytics/azure/ai/textanalytics/_models.py#L872-L879

Generate documentation using tox to verify the docs render as expected.