Fix documentation about hooks

[cabol]

Options :pre_hooks_strategy and :post_hooks_strategy were removed, now the strategy is passed along with the list of hooks, like so:

def pre_hooks do
  pre_hook =
    fn
      (result, {_, :get, _} = call) ->
        # do your stuff ...
      (result, _) ->
        result
     end

  {:async, [pre_hook]}
end

Fix the documentation in Nebulex.Hook module and also in the hooks.md guide – perhaps, check out other possible places.