cmd: remove --help headers and tweak docs.

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Library/Homebrew/cmd/cat.rb

Library/Homebrew/cmd/cleanup.rb

Library/Homebrew/cmd/command.rb

Library/Homebrew/cmd/commands.rb

Library/Homebrew/cmd/config.rb

Library/Homebrew/cmd/deps.rb

Library/Homebrew/cmd/desc.rb

Library/Homebrew/cmd/diy.rb

Library/Homebrew/cmd/doctor.rb

Library/Homebrew/cmd/fetch.rb

Library/Homebrew/cmd/gist-logs.rb

Library/Homebrew/cmd/home.rb

Library/Homebrew/cmd/info.rb

Library/Homebrew/cmd/install.rb

Library/Homebrew/cmd/leaves.rb

Library/Homebrew/cmd/link.rb

Library/Homebrew/cmd/list.rb

Library/Homebrew/cmd/migrate.rb

Library/Homebrew/cmd/missing.rb