Improve `upload_dir` command help strings

[const-cloudinary]

Brief Summary of Changes

This PR started from the review of: #63

@jackieros suggested to improve doc strings of --folder parameter (written in the comment to that PR).

In addition this PR shortens the doc string of the newly added --exclude-dir-name parameter( as @jackieros suggested)

I still think those doc strings are a bit too long and are becoming overcomplicated when writing the difference between dynamic folder environment vs classic.

I'm pretty sure 99% of our customers have no idea what that thing is about.

I would suggest to write a guide/article/reference that would explain how to work with dynamic folders in one page. And if dynamic folders will be our default approach, so the opposite, remove legacy doc strings.

In addition --doc option was added, it points to the documentation page, where we can provide all the details.

What does this PR address?

• [ ] GitHub issue (Add reference - #XX)
• [x] Refactoring
• [ ] New feature
• [ ] Bug fix
• [ ] Adds more tests

Are tests included?

• [ ] Yes
• [x] No

Reviewer, please note:

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I ran the full test suite before pushing the changes and all the tests pass.