`nbdev_clean` documentation partly wrong

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!

Start at the linked API documentation entry for nbdev_clean and compare the fname description with the command behavior listed in the issue. Update the documentation to distinguish a single notebook name or path from unsupported multiple-file and glob arguments, then verify the documented examples.

Written by the indexing model from the issue text.