[Docu] Rm. documentation for process_id output.

Documentation/ProjectFile/prj/time_loop/output/t_suffix.md

 Template for the suffix which will be appended to the output files. Allowed
 template expressions are:
 - {:meshname}
-- {:process_id}
 - {:timestep}
 - {:time}.
 
-Default value: ts_{:timestep}_t_{:time}
+Default value: _ts_{:timestep}_t_{:time}
 
 Remark: The name of the result files will be constructed out of the prefix and
 the suffix.