DOC: Added an example of pitfalls when using astype
- [x] closes #13260 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

@pfrcks Thanks for the contribution!

Personally, I would reverse the logic in the explanation, by first saying how you should do it, and only then what does not work.

Eg in the sense of "When you want to astype subset of columns -> you have to do it like this: ... -> attention, you cannot use loc in this case like ... as this upcasts"
 @jorisvandenbossche I have made the cleanup changes you recommended. Do I need to change the order of the wrong and right way of doing things?
Wrong followed by right way of doing things also sends a clear message I guess. We see how the thing is wrong, then we see how to correct or implement it properly.
 Restructured the documentation.
 @jorisvandenbossche Made the suggested changes. Hope it is fine now.
 Only a small last comment, for the rest looks good!
 @jorisvandenbossche I hope that does the trick ;)
 @pfrcks Thanks!
