The return type is critical for understanding how to use a function. In particular with the Java client library, it's critical to know if a function returns a Future (i.e., it's an async function) or not. Otherwise it's impossible to use it correctly.
Currently one must click through, scroll around, and pick out the return value somewhere in a list of parameters on the details page. It would be worth the extra screen real estate on the api reference page to show the return value as well.