Document which variable expansions are available in command aliases

[kurahaupo]

When creating a new command alias, it would be really helpful to know what variables are available and what they expand to, such as:

• a link to a page, perhaps on github.io
• have a pop-up cheat-sheet
• show a menu when the user starts typing ${

If #164 is implemented, then also include an explanation of how they will affect whether an alias is included in the menu.

[MCMrARM]

I think it is documented on https://github.com/MCMrARM/revolution-irc/wiki/Command-aliases Could presumably include an in-app link if there isn't one already.