Document how multiple package repositories are combined

From the latest documentation on package indices / repositories:
https://github.com/haskell/cabal/blob/a5ddb14e0a648ea8608d0de987c417cb17c16441/doc/config.rst?plain=1#L138-L140
This does not say _how_ these indices are combined. A crucial information missing here is the precedence.
If I have several package indices that both define a certain package, which of the indices takes priority?