Add support for making docstrings for commands available

We have the feature already for attributes, so it would be nice to have it for commands as well.

But according to https://tango-controls.readthedocs.io/projects/pytango/en/latest/api/client_api/miscellaneous.html#tango.DevCommandInfo we would need to choose one of out_type_desc/in_type_desc as there is no general documentation field.