Clarified `annotations_path`, `split params` in `project.upload`.

[LinasKo]

Description

This seems like a miniscule change, but it it would have saved me 2 hours today.

It took time to create a COCO annotations file, only to find that:

• RF API fails silently when you send a file with an incorrect format, without feedback as to what's wrong
• The parameter allows sending of the class label directly.

List any dependencies that are required for this change.

None

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] This change requires a documentation update

This only changes the docstring.

How has this change been tested, please provide a testcase or example of how you tested the change?

N/A

Any specific deployment considerations

~~I got mkdocs serve running, but could not locate any Project docstring renderings. If that is displayed somewhere, it should be checked that parameters of Project are displayed correctly.~~

I think it's fine.

Docs

• [x] Docs updated? What were the changes:

Clarified a special case of annotations_path, explained it more thoroughly, and clarified the explanation for split.