Actually document the JSON command structures.

[madolson]

As per, https://github.com/placeholderkv/placeholderkv/pull/35/files#diff-cccaa466d7e18f8b5fdf9fe8b0d2abd4b29d39f509af8bdb8d3878a0fbf02ac2L11-L12.

The structure of each JSON is somewhat documented in https://redis.io/commands/command-docs/ and https://redis.io/commands/command/

We should probably actually document them for external and internal users.