Thank you.

I think the specific list of list example is better for the sum documentation because lists support the + operator. I don't think
that someone would consider using sum for chaining arbitrary iterables.
What about a concise "To concatenate lists use itertools.chain.from_iterable." and letting the "apply list constructor" part as an exercise to the reader?

I did the explicit reference to itertools.itertools.chain.from_iterable since the default or the explicit link to itertools.chain.from_iterable did not produce a correct link in the html docs.
I applied patch3 and ran make html; there is no html link to from_iterable produced.