Document how to correctly create Channel and User instances

Document that using Channel.new and User.new aren't the correct way of
obtaining instances of these classes, and that one should use
ChannelList and UserList instead.

Thanks to Simon Lundström for an attempted patch.

Closes gh-203

lib/cinch/channel.rb

@@ -43,6 +43,9 @@ module Cinch
     # @return [Hash{String => Object}]
     attr_reader :modes
     synced_attr_reader :modes
+
+    # @note Generally, you shouldn't initialize new instances of this
+    #   class. Use {ChannelList#find_ensured} instead.
     def initialize(name, bot)
       @bot    = bot
       @name   = name

lib/cinch/user.rb

@@ -185,7 +185,8 @@ module Cinch
     # @api private
     attr_writer :monitored
 
-
+    # @note Generally, you shouldn't initialize new instances of this
+    #   class. Use {UserList#find_ensured} instead.
     def initialize(*args)
       @data = {
         :user         => nil,