Directly redirecting prefix location to named one

[B.R.]

I am considering the following locations:

location / {
    proxy_pass http://upstream;
}

location /documents/ {
    try_files $uri @upstream;
}

location @upstream {
    proxy_pass http://upstream;
}

I would like to have a single named location to handle all fallbacks to
upstream (to avoid duplication: maintenance will be easier!).

How does one redirect a prefix location directly to a named one?
Using try_files might expose documents which are not supposed to be served
outside of the /documents/ tree.

Using rewrite? I learned here to avoid it as much as possible...
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*B. R.*
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