Ok, if you already have RapidMiner inside eclipse it makes things way easier
The RapidMiner core contains a file called OperatorsCoreDocumentation.xml which contains the documentation for all operators. Here you can search for the name of the operator and then find its key.
Then open OperatorsCore.xml and search for the key. Then you'll find the class name of the operator next to the key.
For operators from extensions you have to also checkout the extension and then search for the file Operators[NameOfPlugin]Documentation.xml