Document `beam_pipeline_options`

[ethanabrooks]

Description of issue

It is not clear to me from the documentation what inputs are valid for the tfds build --beam_pipeline_options flag. In particular, I am interested in figuring out how to execute the build in a Docker container per https://beam.apache.org/documentation/runtime/environments/#custom-containers (perhaps this should be a separate issue).

I am happy to submit a PR, but I don't know the answer to my own question.