load_commands: Clarify documentation
load_commands: Clarify documentation

Fix formatting on `LoadCommand`'s toplevel comment, and clarify
the meaning of `LoadCommand#cmd` and the return value of
`LoadCommand#type`.

Homebrew/ruby-macho,Homebrew/ruby-macho,woodruffw/ruby-macho,Homebrew/ruby-macho,woodruffw/ruby-macho,woodruffw/ruby-macho