Fix documentation example of fluent helpers

The REPL output now matches the instructions. Extra info has been added
to aid with the explanation as well for those who are not aware of DST.

Author: oliverw1

docs/docs/fluent_helpers.md

@@ -45,23 +45,24 @@ converts the time in the appropriate timezone.
 ```python
 >>> import pendulum
 
->>> dt = pendulum.datetime(2013, 3, 31, 2, 30)
->>> print(dt)
+>>> dt0 = pendulum.datetime(2013, 3, 31, 2, 30)
+>>> print(dt0)
 '2013-03-31T02:30:00+00:00'
 
->>> dt = dt.set(tz='Europe/Paris')
+>>> # Paris started DST on that exact day, so 2h30 local time doesn't exist. It immediately rolls over to 3h30. 
+>>> dt = dt0.set(tz='Europe/Paris')
 >>> print(dt)
 '2013-03-31T03:30:00+02:00'
 
->>> dt = dt.in_tz('Europe/Paris')
+>>> dt = dt0.in_tz('Europe/Paris')
 >>> print(dt)
 '2013-03-31T04:30:00+02:00'
 
->>> dt = dt.set(tz='Europe/Paris').set(tz='UTC')
+>>> dt = dt0.set(tz='Europe/Paris').set(tz='UTC')
 >>> print(dt)
 '2013-03-31T03:30:00+00:00'
 
->>> dt = dt.in_tz('Europe/Paris').in_tz('UTC')
+>>> dt = dt0.in_tz('Europe/Paris').in_tz('UTC')
 >>> print(dt)
 '2013-03-31T02:30:00+00:00'
 ```