> For feature parity with Intellij IDEA and MS VS, we should be able
> to display the list of completions and documentation for the
> currently selected completion in two separate popups:
> https://i-msdn.sec.s-msft.com/dynimg/IC797655.jpeg
> https://www.jetbrains.com/img/webhelp/idea/constructors_docs_in_completion.png

FWIW, Icicles has long provided this, just by using *Completions*
for the completions and *Help* for the current-completion doc.

You can of course use separate frames for these.  There is also an
option that moves the *Completions* frame to the display edge when
you request candidate help or you act otherwise on a candidate.

Users control when/if candidate help is displayed (by key).
It is not just thrown in their face systematically.

Here is a screenshot of one possible layout:
http://www.emacswiki.org/emacs/Icicles_-_A_Propos_d'Apropos#CompFrameMovedLeft

FWIW, I also consider it a plus, not a minus, that the window-mgr
"decorations" are present.  Users can move the frames, iconify
them, shrink/enlarge them - do whatever they want with them,
including in some cases on the fly, by key.

They are ordinary Emacs frames, not just hard-coded popups
that exhibit only some developer's idea of the best behavior.

And you'll notice that the *Completions* mode line is made use
of; it provides brief help on the current candidate even when
*Help* is not shown.

I do agree that things like a menu-bar are best removed, by
default (as shown).  But users can always add them, if they
prefer.