doc: add double border around Neovim popup window

Author: bhcleek

autoload/go/doc.vim

@@ -127,6 +127,7 @@ function! s:GodocView(newposition, position, content, package) abort
             \ 'width': width,
             \ 'height': height,
             \ 'style': 'minimal',
+            \ 'border': 'double',
             \ }
       call nvim_open_win(buf, v:true, opts)
       setlocal nomodified nomodifiable filetype=godoc