Enhanced guess_bounds docstring.

From user-raised issue

> I've got an issue with a guess_bounds call creating garbage bounds data for an extracted cube which crosses the longitude boundary of the original field...
I'm happy that guess_bounds is behaving as might be expected, but I would argue that the documentation is insufficient; a note describing what happens when guess_bounds is used in a case such as this one would be valuable, along with some indication of how to modify the bounds by hand.

(ref: WO0000000053479 )

I think this needs no CHANGES/WhatsNew entry ?