Document individual admin commands in markdown

The "ZooKeeper Commands" section of the ZooKeeper Administrator's Guide takes time to document each four letter command individually but when it comes to the admin commands, it just directs the user to query a live peer in order to get the supported list (e.g. curl http://localhost:8080/commands). While such a query will provide the best source for the admin commands available on a given ZooKeeper version, it's not replacement for the role that the central guide provides.

Create an enumerated list of the supported admin commands in the section "The AdminServer" in the style that the four letter commands are documented in "The Four Letter Words".