[minor] update docstring for TemplateJob

[samwaseda]

There was a huge confusion yesterday as it was not clear how to use a custom job defined in TemplateJob so I updated the doc string.

[jan-janssen]

I like the update to the docstring, I am just a bit surprised that people still use the TemplateJob class directly. I thought we replaced that by wrap_python_function() and create_job_class() https://pyiron-base.readthedocs.io/en/latest/example.html

[samwaseda]

One case where people want to use TemplateJob is when they want to use a variable for both input and output. In that case it makes more sense to define it in __init__ and use it in write_input and collect_output.

But one way or other, as long as the class is there we should make it user-friendly.