clarify AJAX wait behaviour example in README

I explained with some more details to the `!has_xpath/has_no_xpath` differences. 

I tried to use simple english and to

README.md

@@ -795,13 +795,23 @@ Be aware that because of this behaviour, the following two statements are **not*
 equivalent, and you should **always** use the latter!
 
 ```ruby
-!page.has_xpath?('a')
-page.has_no_xpath?('a')
+# consider a page where the `a` tag is removed through AJAX after 1s
+visit(some_path)
+!page.has_xpath?('a')  # is false
+page.has_no_xpath?('a')  # is true
 ```
 
-The former would immediately fail because the content has not yet been removed.
-Only the latter would wait for the asynchronous process to remove the content
-from the page.
+First expression: 
+- `has_xpath?('a')` is called right after the page visit. It is `true` because the link has not yet been removed
+- Capybara does not wait upon successful assertions, therefore **it returns `true` right ahead**
+- The expression returns `false` (because it is inverted with the leading `!`)
+
+Second expression:
+- `has_no_xpath?('a')` is called right after the page visit. It is `false` because the link has not yet been removed. 
+- Capybara waits upon failed assertions, therefore **it does not return `false` right ahead** 
+- Capybara will periodically re-check the assertion up to the `default_max_wait_time` defined
+- after 1s, the assertion becomes `true` (because the link has been removed)
+- The expression returns `true`
 
 Capybara's RSpec matchers, however, are smart enough to handle either form.
 The two following statements are functionally equivalent: