Improve Custom Rules documentation
Improve Custom Rules documentation

- Document `/chall USER, FORMAT @@ RULES`

- Document complex bans

- Document multiple rules, in case it wasn't obvious from the rest of
  the page

urkerab/Pokemon-Showdown,sirDonovan/Pokemon-Showdown,xfix/Pokemon-Showdown,Enigami/Pokemon-Showdown,Zarel/Pokemon-Showdown,urkerab/Pokemon-Showdown,Zarel/Pokemon-Showdown,xfix/Pokemon-Showdown,sirDonovan/Pokemon-Showdown,urkerab/Pokemon-Showdown,sirDonovan/Pokemon-Showdown,svivian/Pokemon-Showdown,xfix/Pokemon-Showdown,svivian/Pokemon-Showdown,Enigami/Pokemon-Showdown,xfix/Pokemon-Showdown,svivian/Pokemon-Showdown,Enigami/Pokemon-Showdown,Enigami/Pokemon-Showdown,svivian/Pokemon-Showdown,xfix/Pokemon-Showdown,AustinXII/Pokemon-Showdown,Zarel/Pokemon-Showdown,AustinXII/Pokemon-Showdown,svivian/Pokemon-Showdown,AustinXII/Pokemon-Showdown