CLN/DOC: cache_readonly: remove allow_setting + preserve docstring
Closes https://github.com/pandas-dev/pandas/issues/18197

This kind of 'works' in that `?` or `__doc__` now give the correct docstring, but it's not yet fully ideal as it is now seen as a class (which will probably mess up sphinx' autodoc/autosummary, which I didn't check yet)