`nbdev_clean` documentation partly wrong

[ritagonmar]

I was trying out nbdev_clean and I realized the documentation is not completely right, which caused me some confusion.
In the documentation (https://nbdev.fast.ai/api/clean.html#nbdev_clean) it says:

fname str None A notebook name or glob to clean

but this is not true, it accepts a single notebook name or a path, but not a glob (see detailed below). Maybe this should be changed to avoid confusion, cause it took me a while to understand what works and what doesn't, and I only figured with the help of old issues.

what works:

• one single file (e.g. nbdev_clean --fname nb1.ipynb)
• a path (e.g. nbdev_clean --fname ./)

what does not work:

• passing several files (e.g. nbdev_clean --fname nb1.ipynb nb2.ipynb)
• a glob (e.g. nbdev_clean --fname *.ipynb)
• a path with a glob (e.g. nbdev_clean --fname ./*.ipynb)

Very cool library and features otherwise!