bpo-36260: Add pitfalls to zipfile module documentation (#13378)

* bpo-36260: Add pitfalls to zipfile module documentation

We saw vulnerability warning description (including zip bomb) in Doc/library/xml.rst file.
This gave us the idea of documentation improvement. 

So, we moved a little bit forward :P
And the doc patch can be found (pr).

* fix trailing whitespace

* 📜🤖 Added by blurb_it.

* Reformat text for consistency.