Missing documentation for the "json" communication mode of the Actions plugin

[jrohel]

The Actions plugin supports two modes of communication between the process being executed and the plugin:

• "mode=plain" - The original and default communication model.
  Data is only passed to the process via arguments when it is started. The process can send data to the actions plugin by writing to the standard output.

• "mode=json" - Communication mode introduced in version 1.2.0 of the actions plugin.
  This mode adds bidirectional communication between the actions plugin and the process. This is a request/response type of communication. The process sends requests and the actions plugin sends responses. The process writes requests to the standard output and reads responses from the standard input. The communication uses JSON format.

The "json" communication mode offers more functionality, but is not documented.