Document LOOPBACK environment type

• Right now, LOOPBACK is not mentioned as a possible option for environment_type [1]. It seems that it was intended for testing [2], but it's useful for getting started on Beam and debugging as well, so it's worth a mention.
• The meaning of each environment type should be documented somewhere, such as on the runner pipeline options tables on the website.

[1] https://github.com/apache/beam/blob/master/sdks/python/apache_beam/options/pipeline_options.py#L818-L819

 [2] https://github.com/apache/beam/blob/master/runners/core-construction-java/src/main/java/org/apache/beam/runners/core/construction/Environments.java#L82