Fix some documentations & comments (#5286)

* add comments for the new reward filters

* slightly improve documentation of r9k values

src/controllers/filters/lang/Filter.cpp

@@ -50,6 +50,9 @@ ContextMap buildContextMap(const MessagePtr &m, chatterino::Channel *channel)
      * message.content
      * message.length
      *
+     * reward.title
+     * reward.cost
+     * reward.id
      */
 
     using MessageFlag = chatterino::MessageFlag;