src/libexpr/eval.hh: add link for allowed-uris option

This commit adds a link to the documentation for `--option
allowed-uris` where that option is mentioned while describing
`restrict-eval`.

src/libexpr/eval.hh

@@ -741,7 +741,8 @@ struct EvalSettings : Config
           If set to `true`, the Nix evaluator will not allow access to any
           files outside of the Nix search path (as set via the `NIX_PATH`
           environment variable or the `-I` option), or to URIs outside of
-          `allowed-uris`. The default is `false`.
+          [`allowed-uris`](../command-ref/conf-file.md#conf-allowed-uris).
+          The default is `false`.
         )"};
 
     Setting<bool> pureEval{this, false, "pure-eval",