Clean up doc string

[ablaom]

https://github.com/pebeto/MLJFlow.jl/blob/70e961e9244645b4dfc52a99eab06aea0852f0b1/src/types.jl#L9

The first part of the docstring should address the casual user. So your "Fields" section should not mention client at all (this is part of the implementation, not the user interface) and somewhere you should explain the meaning of the baseuri argument.

(Implementation details generally go in code comments, unless you are providing a method that is part of a public interface, such as our log_evaluation method, in which case put that at the end, possibly under a separate "New implementations" section.)