Explain why the session lives in a per-instance slot

Co-authored-by: Danny Kim <temeddix@gmail.com>
Co-committed-by: Danny Kim <temeddix@gmail.com>

src/lib.rs

@@ -32,6 +32,10 @@ struct Session {
   signer: Signer,
 }
 
+// The host creates one component instance per connection and calls into it
+// on a single thread, so this slot holds exactly one connection's session and
+// is not shared state between connections. A WASI component has no other place
+// to keep state between exported calls.
 thread_local! {
   static SESSION: RefCell<Option<Session>> = const { RefCell::new(None) };
 }