Section on using secure repositories is incomplete

**What is wrong with the docs?**

https://cabal.readthedocs.io/en/stable/config.html#using-secure-repositories

At the end of this section, speaking about how to find trusted root keys, the document states "How that is done is... outside the scope of cabal proper." This sentence speaks to a certain familiarity with how these things are supposed to work, and subtly hints that there's probably a more general problem here that could be solved in "the usual ways". But... I certainly don't know how to go about it. Do you? Does anybody? Some pointers would be helpful.

Start with the “Using secure repositories” section at config.html#using-secure-repositories, especially the sentence about finding trusted root keys. Review the surrounding Cabal documentation and the secure-repository implementation or references to determine what practical pointers can be documented. Done means the section explains where users can find or configure trusted root keys without relying on unexplained familiarity.

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