Add some documentation on gotchas related to pytz updates #7620
closes #7620 

can u make this pytz/dateutil (or just say tz library)

as this is a function. of the time zone definitions and not the tz library itself
 Done.

Also, the ipython slowness was actually due to how completion is implemented in PyDev.  It does dir(object) and then iterates through the attributes calling getattr in order to call inspect.getdoc.  With a large DatetimeIndex, getattr for properties evaluates the property and some of those take 15 seconds.  Trying to work out a solution to that on the PyDev side.
 @rockg, if that's of any help, I've recently pushed a [fix](https://github.com/ipython/ipython/pull/4827) into ipython introspection utilities to avoid evaluating properties in the last step of the lookup when doing question-mark queries, e.g. `obj.this_is_evaluated.this_is_not?` It ended up pretty simple, maybe you can borrow some inspiration from there.
 @jorisvandenbossche ?
 looks ok. Does this close #7620 ?
 @immerrr Thank you for the reference.  I was doing something similar.  I simply did:

```
try:
   obj = getattr(type(parent_obj), attr)
except AttributeError:
   obj = getattr(parent_obj, attr)
except:
   continue
```
