Updated doc patch 0001-2.diff: following ncoghlan's request, the bulk of yield documentation is kept in expressions.rst, and simple_stmts.rst mostly refers to the other one. (In previous version it was the other way around).

After doing this change, I still think that it is better to have most of the documentation in the _statement_ part, and keep the _expression_ part relatively simple. The reason is that yield does more than just return a value, but also throws exceptions, suspends execution, etc. This is detached from the fact that is can be used in an expression.

Unfortunately yield as an expression is the only case where a statement can be used in an expression. Therefore there aren't any other cases which could be used as a guide.