Add do#show_documentation() using coc.nvim

Change the `K` normal mode overrides to use the example given in the
coc.nvim README file which uses Vim's builtin help when in `vim` or
`help` files, or checks if coc.nvim is ready and invokes
`CocActionAsync('doHover')`, otherwise fallback to using `keywordprg`.

autoload/do.vim

@@ -100,3 +100,14 @@ endfunction
 " A sink for mappings to do nothing
 function! do#sink() abort
 endfunction
+
+" Used by normal mode K mapping to show documentation
+function! do#show_documentation()
+  if index(['vim','help'], &filetype) >= 0
+    execute 'help '.expand('<cword>')
+  elseif coc#rpc#ready()  " TODO: Check if the LS supports doHover
+    call CocActionAsync('doHover')
+  else
+    execute '!'.&keywordprg.' '.expand('<cword>')
+  endif
+endfunction