It's a side effect of the hidden closure that provides the new scope for the iteration variable - that's an ordinary function object, so using yield or yield from turns it into a generator expression instead. Generator expressions are already generators, so using yield or yield from just adds more yield points beyond the implied ones.

I've never figured out a good way to document it - it's a natural consequence of the comprehension's closure. An explicit mention in the reference docs for comprehensions may be worth adding.