Improve output documentation

[d4straub]

Output documentation needed a review. Here it is.

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!
• If necessary, also make a PR on the nf-core/mag branch on the nf-core/test-datasets repo
• 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
• README.md is updated

[d4straub]

Since local linting and linting in my repo passes, I'll trigger it here once more.

[skrakau]

Great! Here a few comments

[skrakau]

Looks good to me