Document/create update hook for blacklisted paths

ludo.r → made their first commit to this issue’s fork.

Hello,

I just created a merge request with an update hook that does that (patch also provided).
It will first look if file_assets_path is set and it not, fall back to file_public_path.

Feel free to add documentation update.

However, I'm wondering, if this path should always be blacklisted, then why not add it dynamically in-code?
In that way we don't rely on (possibly wrong) configuration.