Remove invalid advice

Capybara's Readme clearly says:
```
The two following statements are functionally equivalent:

page.should_not have_xpath('a')
page.should have_no_xpath('a')
```

Both of them do not wait if there is no element since [Capybara overrides `does_not_match?` too](https://github.com/jnicklas/capybara/blob/master/lib/capybara/rspec/matchers.rb#L22)