I don’t use CoC and I don’t know every Lua API function so this might be clunky, but the following works on my machine:

function _G.show_documentation()
  local ft = vim.bo.filetype
  local cword = vim.fn.expand('<cword>')
  if ft == 'help' or ft == 'vim' then
    vim.fn.execute('help ' .. cword)
  --[[ elseif vim.fn['coc#rpc#ready']() then
    vim.fn.call('CocActionAsync', {'doHover'}) ]]
  else
    vim.fn.execute(vim.o.keywordprg .. ' ' .. cword)
  end
end

vim.api.nvim_set_keymap('n', 'K', "<Cmd>call v:lua.show_documentation()<CR>", {noremap = true, silent = true})

Note that I commented out the elseif statement since I don’t have CoC and the function fails if it can’t call coc#rpc#ready.