Add another non-public comment on Spanned trait

The whole surrounding module is already commented "not public API" but
in my experience people always find a way to miss that kind of thing.

src/spanned.rs

@@ -2,6 +2,7 @@ use crate::ToTokens;
 use proc_macro2::extra::DelimSpan;
 use proc_macro2::{Span, TokenStream};
 
+// Not public API other than via the syn crate. Use syn::spanned::Spanned.
 pub trait Spanned: private::Sealed {
     fn __span(&self) -> Span;
 }