Offset: simplify jQuery#offsetParent method

* It seems, check for html element (and previously for body element)
  was redundant

* Simplify "return" statement

* Add comment about potential errors that didn't find themselves in real life app

This code would work even in IE6 quirks mode
If these are pathological cases that don't occur in real life, I am not sure why we are commenting them anyway. The result might change in a refactoring and these comments would no longer be true, but people might have read them as documented and supported.