docs on using closure for `path` param in publishDir

New feature

The publishDir docs do not state that a closure is needed for the path parameter if a nextflow parameter is used.

My case:

profiles {
    dev {
        params {
            outdir_prefix     = 'tmp/results'
        }
}

process{
    withName: 'STAR_map' {
        publishDir = [
            mode: 'copy',
            overwrite: true,
            path: { params.outdir_prefix },
            pattern: '*'
        ]
    }
}

Without the closure ({ params.outdir_prefix }), I get the error:

Target path for directive publishDir cannot be null

It took quite a bit of time to figure out that a closure is needed.

Start with the publishDir documentation section linked in the issue and review the provided configuration example showing params.outdir_prefix. Update the documentation to explain when the path parameter requires a closure, using the reported null-target-path behavior as context; done means users can find this guidance there.