docs: add documentation

Add documentation on the grpc calls that were still missing it.

engine-protocol/src/main/proto/engine_control.proto

@@ -34,15 +34,27 @@ message WaitForIdleStateResponse {}
 message GetRecordsRequest {}
 
 message RecordResponse {
+  // A JSON representation of a Record.
   string recordJson = 1;
 }
 
 service EngineControl {
 
+  /*
+    Starts the in-memory engine.
+   */
   rpc StartEngine (StartEngineRequest) returns (StartEngineResponse);
 
+  /*
+    Stops the in-memory engine.
+   */
   rpc StopEngine (StopEngineRequest) returns (StopEngineResponse);
 
+  /*
+    Resets the in-memory engine. This is done by recreating the engine entirely.
+    The engine should be reset before executing the next test. This prevents
+    the tests from influencing each other.
+   */
   rpc ResetEngine (ResetEngineRequest) returns (ResetEngineResponse);
 
   /*
@@ -58,7 +70,10 @@ service EngineControl {
    */
   rpc WaitForIdleState (WaitForIdleStateRequest) returns (WaitForIdleStateResponse);
 
-
+  /*
+    Get all records from the in-memory engine. These records will be returned
+    in a JSON format. Client-side these should be mapped to Records.
+   */
   rpc GetRecords (GetRecordsRequest) returns (stream RecordResponse);
 
 }