Add doc comment help

[rjmholt]

I feel like we used to have this, but either way, we should display comment-based help for commands in the completion popup.

[rjmholt]

Looking into this, I don't believe we've ever supported this.

It should be relatively straightforward to accomplish with Get-Help -- we should just make sure to cache the results.

[iTiamo]

Yes please. I would benefit massively from this.