[T5 Conf] rename docstring to acuatly argument names
As mentioned in issue: #4139, the docstring names of the T5 Config are confusing since those names cannot be used to set the arguments. 

This PR renames the arguments in the docstring and adds an explanation that those arguments can also be accessed via their properties. 

To not break backward compatibility, renaming the docstring is better than renaming the actual variables IMO. 