message: docs(aio): document the special treatment of undefined
diff: @@ -142,7 +142,7 @@ The `hero` in `{{hero.name}}`
  refers to the template input variable, not the component's property.
  Template expressions cannot refer to anything in
  -the global namespace. They can't refer to `window` or `document`. They
  +the global namespace (except `undefined`). They can't refer to `window` or `document`. They
  can't call `console.log` or `Math.max`. They are restricted to referencing
  members of the expression context.
  