Document PirateGame::Background
Rename Background::STATES to WEATHER

It holds the weather, so let's name it that.
Undent PirateGame::Boot

All that wasted whitespace!
Document PirateGame::Boot
Document PirateGame::WavingItem

Update Boot::waving_offset for the :slow speed.
Improve WavingItem#waving_offset documentation
Document PirateGame#Wave