Clarify that you must use publish: to produce output files with new syntax

[adamrtalbot]

The new workflow outputs syntax just suggests you can use an output block to create outputs:

workflow {
    foo(bar())
}

output {
    directory 'results'
}

However, this isn't the case and you must use the publish: directive in the process or workflow syntax to make it work:

workflow {
    foo(bar())

    publish:
    foo.out >> 'foo'
}

output {
    directory 'results'
}

We should clarify the documentation to say this.

[ewels]

Will be fixed by #5185