Playing with the new Set code in nfo, and I see that the overview files is placed exactly where the first movie in the set is listed.
Code:
<set>
<name>The Lord of the Rings Collection</name>
<overview>Classic story by J R Tolkins</overview>
</set>
I suspect this is a Skin issue, as Estuary skin lists the movies in a collection, so this list starts at the same position as the overview is placed.
Tested with confluence and it displays the <overview> correctly, but, confluence skin does not list the movies in a collection.