Update Operator `into` documentation

[harper357]

Not sure if this is the correct place for this, my apologies if it isn't.

The current documentation for the into operator says that it is not longer available in DSL2, but it isn't clear if there is a replacement or if it is no longer needed. After asking in the help Slack, I was informed that it is the later. It would be great if this could be updated to state that or possibly linked to the Channel forking section.

[pditommaso]

You are right, this needs to be addressed. I've initiated a PR to update the docs to DSL2 syntax https://github.com/nextflow-io/nextflow/pull/2816