document store url trusted=true option behavior

src/libstore/globals.hh

@@ -782,6 +782,7 @@ public:
 
           - the store object has been signed using a key in the trusted keys list
           - the [`require-sigs`](#conf-require-sigs) option has been set to `false`
+          - the store URL is configured with `trusted=true`
           - the store object is [content-addressed](@docroot@/glossary.md#gloss-content-addressed-store-object)
         )",
         {"binary-cache-public-keys"}};