docs: more faq

docs/faq.md

@@ -6,7 +6,10 @@
   It's more like Connect, but a lot of the Express goodies
   were moved to the middleware level in Koa to help form
   a stronger foundation. This makes middleware more enjoyable
-  and less error-prone to write. Typically many middleware would
+  and less error-prone to write, for the entire stack, not
+  just the end application code.
+
+  Typically many middleware would
   re-implement similar features, or even worse incorrectly implement them,
   when features like signed cookie secrets among others are typically application-specific,
   not middleware specific.