Document all short flags.

Library/Homebrew/cmd/fetch.rb

Library/Homebrew/cmd/install.rb

Library/Homebrew/cmd/link.rb

Library/Homebrew/cmd/migrate.rb

Library/Homebrew/cmd/outdated.rb

Library/Homebrew/cmd/uninstall.rb

Library/Homebrew/cmd/unpack.rb

Library/Homebrew/cmd/update.sh

Library/Homebrew/dev-cmd/bottle.rb

Library/Homebrew/dev-cmd/test.rb

Library/Homebrew/dev-cmd/tests.rb

docs/Manpage.md

manpages/brew-cask.1

manpages/brew.1