append/prepend are ASYNC, and can specify 0 for casunique

This is a two-for-one bug report for the docs. First, the return value of append/prepend is Future<Boolean>, not "Object ( Binary object )". These are async ops, and need to be documented as such.

Next, the append and prepend methods say that the casunique argument must be specified, but don't mention that it is OK to pass 0 here if you just want to append to whatever is there.

In fact, using 0 for casunique is going to be the normal use case for these functions, since memcached (and Couchbase) defines them to be atomic. It's too bad that spymemcached made this interface clunky, but we can at least document this normal use case.