Minor doc point.

[ablaom]

https://github.com/pebeto/MLJFlow.jl/blob/951fb8f8b03727ec2539b8e9ec832cf44503d345/src/types.jl#L11

To keep doc strings concise, I usually suppress statements like "If the client is not running an informative error will be thrown". The user will find this out if they forget. Otherwise, they don't need this information.