Adds HelpDesc to Command, a more detailed version of HelpText, which may...

... be used by Devs to create a functional multi-line /help <command> description. If HelpDesc is given a value, HelpText will not show.

This will not break any of the current plugins, since it's entirely optional. Adds variety, removes nothing.

If this were to be universalized, players would use built-in /help <command> for every plugin instead of browsing specific plugin help commands, making everything much simpler.

*Thanks for the help with Pull Requests Olink, this is what happens when github fails you.*