Document that wrapping an empty `std::function` creates a non-empty `AnyInvocable`

PiperOrigin-RevId: 522411202
Change-Id: Ifc99f5f6a227efd697039344ea75f53c6f282e53

absl/functional/any_invocable.h

@@ -269,6 +269,14 @@ class AnyInvocable : private internal_any_invocable::Impl<Sig> {
   // absl::AnyInvocable::operator bool()
   //
   // Returns `true` if `*this` is not empty.
+  //
+  // WARNING: An `AnyInvocable` that wraps an empty `std::function` is not
+  // itself empty. This behavior is consistent with the standard equivalent
+  // `std::move_only_function`.
+  //
+  // In other words:
+  //   std::function<void()> f;  // empty
+  //   absl::AnyInvocable<void()> a = std::move(f);  // not empty
   explicit operator bool() const noexcept { return this->HasValue(); }
 
   // Invokes the target object of `*this`. `*this` must not be empty.