Simplify docstrings where no additional information added

[aaron-hopkinson]

There are some instances where we can simplify the docstrings to reduce clutter in the code, e.g. private methods where the method name combined with type hints are self-documenting. In these cases, we should reduce the long-form docstring to a single line one. (See: https://github.com/ecmwf/anemoi-transform/pull/146/files#r2266089647)

This issue covers going through the code and reducing the verbosity of the docstrings where appropriate.