Updating the API guide with a few more pitfalls. (#1059)
* added some pitfalls
* bulleting correct
* made code block python
* Instead of `xm.mark_step`, mention `barrier=True`.
* Added barrier=True in the markdown as well as the code block.
* Address Alex s comments.
* "There is no steps" corrected to "There are no steps."
* Addressing Davide s comments.
* Inlined torch.where per suggestion.
