Fix redundant UDG flags

[apeltzer]

This addresses #89 directly, also documenting the changes in both changelog and user documentation.

PR checklist

• This comment contains a description of changes (with reason)
• If you've fixed a bug or added code that should be tested, add tests!
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Make sure your code lints (nf-core lint .).
• Documentation in docs is updated
• CHANGELOG.md is updated

[jfy133]

What happens if --udg is set but PMD is not turned on downstream? What other tools utilise this flag?Thinking from a (novice-)user perspective, they might turn that flag on even if they don't want PMD tools.

Secondly, I would rather say --UDGnone instead of --UDGminus. I think this is clearer - at least from the terminology in our institute (i.e. none/half/full).

There is a typo which I've marked in a code comment.