add missing docstrings to public properties in loggers

[awaelchli]

📚 Documentation

There are a few undocumented public properties in the logger wrappers.
Example:
https://github.com/PyTorchLightning/pytorch-lightning/blob/92e49795e1e044ee7f1ca450babf0c50c1b5d81f/pytorch_lightning/loggers/mlflow.py#L174

Let's document these so they show up in our HTML docs and so users can discover them.

This is a good issue for new contributors!! If you are interested, please send a pull request right away (one per logger class).

[bamblebam]

I can do this

[bamblebam]

I think this should still be open as the other logger classes are left.

[awaelchli]

@bamblebam it was auto-closed. It was because you had "Fixes xyz" in the description of the PR, GitHub was smart about it to close after merge :)

[bamblebam]

Since the doc strings in other logger classes have been updated is this issue done or do we need to update the base logger class as well.

[awaelchli]

Thanks a lot.
Feel free to update the base too if you see anything important missing, but I think most of it is covered :)