If you've just read Paul Cutler's blog post about Mallard documentation and want to get involved, consider translating some of the GNOME Games help!
The games are a good project to start with because their help is relatively simple.
I've opened bugs against Sudoku, Mines, Mahjongg and Quadrapassel (Tetris), leave a comment if you start working on it.
To get started look at the existing documentation either by running the games or browsing the help in git. Learn about Mallard by doing the ten minute tour.