make new user experience smoother

This adds some noise, which I'm not a huge fan of. However if I hadn't
had my cup of coffee and not tried reading the type error, I might have
pushed back "actually use polysemy in the real world" to "later".

Ideally I think I would have read the docs, but I suspect that "paste
examples verbatim into ghc source file without reading docs" is a
popular method to try libraries out.

Fixes polysemy-research#349

src/Polysemy/Internal.hs

@@ -113,6 +113,7 @@ import Polysemy.Internal.Union
 --   example
 --     & runError
 --     & fmap (either id id)
+--     -- Without `-fplugin=Polysemy.Plugin` you need `evalState @String`
 --     & evalState ""
 --     & runM
 --     & (print =<<)
@@ -123,6 +124,7 @@ import Polysemy.Internal.Union
 --
 -- >>> :{
 --   example
+--     -- Without `-fplugin=Polysemy.Plugin` you need `evalState @String`
 --     & evalState ""
 --     & runError
 --     & fmap (either id id)