[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/PIDCommand.h

@@ -20,6 +20,8 @@ namespace frc2 {
  * The controller calculation and output are performed synchronously in the
  * command's execute() method.
  *
+ * This class is provided by the NewCommands VendorDep
+ *
  * @see PIDController
  */
 class PIDCommand : public CommandHelper<CommandBase, PIDCommand> {