Definition of Report Server Database Table

No I do not believe any official information on these tables exists. My understand is that these tables are considered to be "internal" to the product and officially Microsoft does not support directly querying these tables. On a busy system you could potentially cause locking conflicts which could interfere with the operation of the Report Server.

If you want to interact with the Report Server programatically there are a number of web service & rest based api's that you can use. Or you could use the powershell cmdlets (which sit over the top of these web based api's) see https://docs.microsoft.com/en-us/power-bi/report-server/developer-handbook-overview for more information on the supported ways of talking to Power BI Report Server.