Add examples to man page or --help output

[Jaakkonen]

The grim - argument is easy to forget and swappy just silently fails and it's easy to run swappy - where you mean swappy -f -. The working examples are in README.md but I think it would be nice to have those in man page examples section or even in the swappy --help output.

[jtheoof]

I really don't think that the man page is the place to document non-dependency of tools (like grim or slurp). Swappy can be used outside of the sway ecosystem.