title: YouCompleteMe GetDoc preview window does not trigger on selection
body: <p>As far as I understand YouCompleteMe comes by default with this handy preview window showing whatever the <a href="https://github.com/Valloric/YouCompleteMe#the-getdoc-subcommand" rel="nofollow noreferrer">GetDoc</a> subcommand returns whenever you select a completion. </p>
  
  <p>The issue I'm currently facing with it however is that while it works perfectly fine on Python files, it doesn't trigger at all on Rust files. To clarify I'm merely talking about the preview window showing the documentation here, the autocompletions pop up just fine.</p>
  
  <p>I've looked through the documentation and confirmed that <a href="https://github.com/Valloric/YouCompleteMe#the-getdoc-subcommand" rel="nofollow noreferrer">GetDoc</a> supports Rust. I verified that this is indeed working by manually triggering the command using <code>:YcmCompleter GetDoc</code> which returned the appropriate documentation just fine.</p>
  
  <p>While searching for ways to fix this and have it automatically trigger I have also encountered <a href="https://github.com/Valloric/YouCompleteMe#the-gycm_add_preview_to_completeopt-option" rel="nofollow noreferrer"><code>let g:ycm_add_preview_to_completeopt</code></a> so I included that in my <code>.vimrc</code> just to make sure. Reading up on <code>completeopt</code> I checked what <code>:set completeopt?</code> returns: <code>completeopt=preview,menuone</code>. That looks fine to me as well.</p>
  
  <p>At this point I'm rather lost and my google-fu is also exhausted. What am I missing here? If I'm missing some important information please let me know.</p>