[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/button/POVButton.h

@@ -12,6 +12,8 @@ namespace frc2 {
  * A class used to bind command scheduling to joystick POV presses.  Can be
  * composed with other buttons with the operators in Trigger.
  *
+ * This class is provided by the NewCommands VendorDep
+ *
  * @see Trigger
  */
 class POVButton : public Button {