[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/PIDSubsystem.h

@@ -13,6 +13,8 @@ namespace frc2 {
  * A subsystem that uses a PIDController to control an output.  The controller
  * is run synchronously from the subsystem's periodic() method.
  *
+ * This class is provided by the NewCommands VendorDep
+ *
  * @see PIDController
  */
 class PIDSubsystem : public SubsystemBase {