doc: rewrite the multi-user documentation to actually talk about security

It's in the security section, and it was totally outdated anyway.

I took the opportunity to write down the stuff we already believed.

Change-Id: I73e62ae85a82dad13ef846e31f377c3efce13cb0

1 files changed, 11 insertions, 1 deletions

diff --git a/src/libfetchers/fetch-settings.hh b/src/libfetchers/fetch-settings.hh
index 2dc2834fb..6fb260c3a 100644
--- a/src/libfetchers/fetch-settings.hh
+++ b/src/libfetchers/fetch-settings.hh
@@ -87,7 +87,17 @@ struct FetchSettings : public Config
         {}, true, Xp::Flakes};
 
     Setting<bool> acceptFlakeConfig{this, false, "accept-flake-config",
-        "Whether to accept nix configuration from a flake without prompting.",
+        R"(
+          Whether to accept Lix configuration from the `nixConfig` attribute of
+          a flake without prompting. This is almost always a very bad idea.
+
+          Setting this setting as a trusted user allows Nix flakes to gain root
+          access on your machine if they set one of the several
+          trusted-user-only settings that execute commands as root.
+
+          See [multi-user installations](@docroot@/installation/multi-user.md)
+          for more details on the Lix security model.
+        )",
         {}, true, Xp::Flakes};
 
     Setting<std::string> commitLockFileSummary{