docs suggestion for building ProxyBackends that massage data

This tripped me up a year ago, so I decided to offer a docs update.

It's an example of using a base class to manage the core value-mangling functions, then implementing the proxy via a subclass that merely defines encode/decode. I also added a sidebar explaining some of the value caching behavior.

fwiw, serializing to msgpack -- despite being wrapped in a pickle dogpile -- is cutting our cache size in half.