Thanks for the post! There is not any specific documentation for the plugin beyond the instructions found in the document and Atlassian Marketplace listing you mentioned, but I would be happy to assist with any specific questions you have about the use of the plugin.
The basic functionality of the plugin once installed and configured is to display the TestRail information for any linked test cases (through the references field) and results (through the defects field) directly in the Jira issue interface. It will create two sections when viewing a Jira issue and will populate the testing data in real time using the TestRail API.
I hope that clarifies things! Let me know if you have any other questions and I’ll be happy to help.