Add missing documentation for winch/gripper plugins

[potaito]

Slipped through the cracks in the previous PR: #308

Addresses the following CI failure over in MAVSDK:

2023-01-31T13:50:06.1606188Z Preprocessing /home/runner/work/MAVSDK/MAVSDK/tools/docs/install/include/mavsdk/plug/home/runner/work/MAVSDK/MAVSDK/tools/docs/install/include/mavsdk/plugins/winch/winch.h:91: warning: Compound mavsdk::Winch::StatusFlags is not documented.
2023-01-31T13:50:06.1606831Z /home/runner/work/MAVSDK/MAVSDK/tools/docs/install/include/mavsdk/plugins/winch/winch.h:28: warning: Compound mavsdk::Winch is not documented.
2023-01-31T13:50:06.1607356Z /home/runner/work/MAVSDK/MAVSDK/tools/docs/install/include/mavsdk/plugins/gripper/gripper.h:28: warning: Compound mavsdk::Gripper is not documented.
2023-01-31T13:50:06.1607877Z /home/runner/work/MAVSDK/MAVSDK/tools/docs/install/include/mavsdk/plugins/winch/winch.h:128: warning: Compound mavsdk::Winch::Status is not documented.
2023-01-31T13:50:06.1608449Z Generating docs for /home/runner/work/MAVSDK/MAVSDK/tools/docs/install/include/mavsdk/plugins/gripper/gripper.h:64: warning: Member GripperAction (enumeration) of class mavsdk::Gripper is not documented.
2023-01-31T13:50:06.1609079Z Generating docs/home/runner/work/MAVSDK/MAVSDK/tools/docs/install/include/mavsdk/plugins/winch/winch.h:64: warning: Member WinchAction (enumeration) of class mavsdk::Winch is not documented.