The documentation of REST API is available in the footer of Themis, via the "Web Service API" link. It is internal to the deck and boarded in every new version of Themis. Thus, the documentation that you find is up-to-date with the version that you use.
To begin with the API, you have to beforehand generate keys for the authentication.
Do not hesitate to contact the support for every request of development or improvement of the REST API!