Add description to log level

We can see this via `--show-plugin-config` after merge #706

lib/fluent/log.rb

@@ -343,6 +343,7 @@ def enable_color(b = true)
   module PluginLoggerMixin
     def self.included(klass)
       klass.instance_eval {
+        desc 'Allows the user to set different levels of logging for each plugin.'
         config_param :log_level, :string, :default => nil, :alias => :@log_level
       }
     end