Improve the proto docs to reflect the connection between the oneof fields

[viacheslav-rostovtsev]

Protos allow some fields to be connected in what essentially are the oneof groups. Assigning one field in the group will essentially prevent others from being assigned and there might be more of these behaviours. Docs currently do not reflect that and they should.

[viacheslav-rostovtsev]

Also: document that proto3_optional fields have their has_foo? methods and that oneofs have to check presence via the oneof fields