[commands] Update Command documentation (NFC) (#3881)

Add reference to which VendorDep the class is included in.
Add missing OldCommands C++ Documentation (copied from Java).

wpilibNewCommands/src/main/native/include/frc2/command/StartEndCommand.h

@@ -19,6 +19,8 @@ namespace frc2 {
  * extending and then retracting a solenoid. Has no end condition as-is; either
  * subclass it or use Command.WithTimeout() or Command.WithInterrupt() to give
  * it one.
+ *
+ * This class is provided by the NewCommands VendorDep
  */
 class StartEndCommand : public CommandHelper<CommandBase, StartEndCommand> {
  public: