Text feature doc is missing details of encoder params (parallel_cnn)

[dantreiman]

https://ludwig-ai.github.io/ludwig-docs/0.5/configuration/features/text_features

We removed the detailed encoder docs here because most of the information is duplicated on the sequence encoder page, but in response to user feedback I think we should restore this detail.

In particular, documentation of the details of the basic encoders (embedding, parallel CNN) should be present on the text feature page.