Update processRequest comment to Doxygen format

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/xrpld/app/main/GRPCServer.h

@@ -206,7 +206,10 @@ private:
         clone() override;
 
     private:
-        // process the request. Called inside the coroutine passed to JobQueue
+        /**
+         * Process the gRPC request. Called inside the CoroTask lambda
+         * posted to the JobQueue by process().
+         */
         void
         processRequest();