[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/CommandScheduler.h

@@ -26,6 +26,8 @@ class Subsystem;
  * commands synchronously from the main loop.  Subsystems should be registered
  * with the scheduler using RegisterSubsystem() in order for their Periodic()
  * methods to be called and for their default commands to be scheduled.
+ *
+ * This class is provided by the NewCommands VendorDep
  */
 class CommandScheduler final : public nt::NTSendable,
                                public wpi::SendableHelper<CommandScheduler> {