I have the same question. There is no clustered.sh, so how is it possible to start a clustered-Memcached-Server?!
Sorry, my bad for not updating the documentation yet.
Starting from Infinispan 8.0, the supported mode to start a clustered server is to use domain mode, for which I temporarily redirect you to WildFly's page.
You can still start a standalone server in clustered mode, by invoking it like this:
bin/standalone.sh -c clustered.xml
Additionally, the -all distribution is a bit of a misnomer since it includes everything but the server.