Bit of clarification; the Web Adaptor is aware of the machines in the site as it polls for them every minute. Once it knows which machines are part of the site, it uses round-robin requests to send traffic to each node; it doesn't prioritize sending traffic to the primary.
In terms of the documentation, a single web adaptor, on a single IIS instance, is a single point of failure, which is why that pattern is not described in the high availability documentation. Load balancers are typically fault tolerant, and if you want IWA, you'd put two web adaptors behind the load balancer.
