Can't read all of a completion item's popup window documentation

[habibalamin]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I'm reading the documentation for a completion item, and I get to the end of the popup window and am arbitrarily prevented from finishing the trail I'm on.

Describe the solution you'd like

It'd be nice to be able to scroll the popup window, or at least focus on it. I guess focusing is possible on hover documentation by simply calling ShowHover twice in a row, but how do I do this with completion item documentation?

Describe alternatives you've considered

I've searched the plugin docs to see if there's a way to get the winid for the popup window so I can at least map keys to send commands to it myself, but no go. I've looked at other LSP plugins, but I think I only saw one that was able to do so, and I don't want to switch to it. I've considered switching to Neovim, and I think the best option I have for now is to use the preview window instead (or even live without this feature for a while before switching to Neovim).