list_jobs limit argument needs clarification or more suitable default value

In `JobClient`, the `list_jobs` function has a `limit` argument, which sets the number of jobs returned in a single request. The default value is is `None` in the library, which means "use the default limit from the backend service". That default value is (I believe) `10`.

https://github.com/facultyai/faculty/blob/master/faculty/clients/job.py#L314

Had a request to use that upstream default value in the library to make the effect of different values clearer. Otherwise interpreting the behaviour of `None` is difficult (for example will there be paging in that case or all records are returned?)