Remove statement about should_not have vs should have_no

This statement may have been true in the past, but now
the two statements are functionally equivalent.

README.rdoc

@@ -495,17 +495,6 @@ is (the default is 2 seconds):
 
     Capybara.default_wait_time = 5
 
-Be aware that because of this behaviour, the following two statements are *not*
-equivalent, and you should *always* use the latter!
-
-    page.should_not have_xpath('a')
-    page.should have_no_xpath('a')
-
-The former would incorrectly wait for the content to appear, since the
-asynchronous process has not yet removed the element from the page, it would
-therefore fail, even though the code might be working correctly. The latter
-correctly waits for the element to disappear from the page.
-
 Capybara's waiting behaviour is quite advanced, and can deal with situations
 such as the following line of code: