Document protobuf

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
I am, investigating if I want to/playing around with, making a ballista client in another language. This in turn requires using the datafusion grpc/protobuf files. Most things are self explanatory, however some, like `Column` have mysterious fields like `string relation` with no explanation of what this should be

**Describe the solution you'd like**
Documentation for some of the more non-obvious fields in the protobuf file, I noticed that usually they have a `string` type

**Describe alternatives you've considered**
I can look and spit through the source code. However, this is a considerable pain and not ideal for understanding _what_ something is instead of _how_ something is constructed. My goal is not to copy paste all the code, I would like to be able to work of the documentation if at all possible.

I understand that this is incredibly niche and not many will benefit from this. However I do think it could benefit integration from other projects into ballista(or just datafusion).

Find the DataFusion protobuf definitions and inspect the Column message, especially the relation field, then trace the relevant source usage to determine the intended meanings of non-obvious fields. Document those meanings in the protobuf definitions and verify that the resulting comments explain how clients should use the fields.

Written by the indexing model from the issue text.