hadrian/doc: Clarify documentation of key-value configuration

hadrian/doc/user-settings.md

@@ -382,7 +382,8 @@ the right names for them:
 - the stage slot, which comes first, can be filled with any of `stage0`,
   `stage1`, `stage2`, `stage3` or `*`; any value but `*` will restrict the
   setting update to targets built during the given stage, while `*` is taken
-  to mean "for any stage".
+  to mean "for any stage". For instance, the above example will affect
+  the linking of the `_build/stage1/bin/ghc` executable.
 - the package slot, which comes second, can be filled with any package name
   that Hadrian knows about (all packages that are part of a GHC checkout),
   or `*`, to respectively mean that the builder options are going to be updated