Update command input/output docs

[rgwood]

Context

Earlier today I was trying to replace part of a pipeline with a helper function. It took me a little while to figure it out, and somehow I bounced off the documentation without fully understanding it.

Changes

• Make it extra clear that the docs are referring to pipeline input/output. This should be obvious on a close read but I think I misunderstood it on a quick skim.
• Rewrite the pipeline input example for engine-q; it no longer works on main because each expects a parameter.

Considerations

I initially tried to keep the exact functionality of the my-echo example but Nushell has changed enough to make that difficult; if you just add an |it| parameter it parses but the function incorrectly returns a list of lists:

So instead I just copied the first example from help each.

[fdncred]

Thanks!