Cmdline completion documentation preview?

[tmillr]

It would be really nice to have something like the which-key plugin, but for the cmdline instead of mappings. Or, in other words, when typing a cmd at the cmdline, after a configurable timeout (e.g. 2000 ms) a doc window/preview opens up showing nvim's help for the typed cmd. Are there certain settings or a plugin which accomplishes this? I couldn't find one. This would be helpful because alot of commands accept arguments, options, or bang, etc. and can have different behavior depending upon these factors. This could be opt-in and would be a helpful feature!

[Shougo]

Unfortunately, cmdline completiongetcompletion() does not have the information.
It is hard.

For example, your command line manual completion Tab prints the information?
Because Vim/neovim has not.

[tmillr]

No neovim does not do this by default either. It might be somewhat difficult to implement but not impossible. All builtin cmdline/ex commands are documented and tagged (usually tagged under the command's name itself), and this plugin already has the capability of displaying docs in a floating window.

[Shougo]

Yes. I think it is not possible. Just hard.