add jsdocs, update API description

JitsiConferenceEvents.js

      * Indicates that conference has been left.
      */
     CONFERENCE_LEFT: "conference.left",
+    /**
+     * Indicates that DTMF support changed.
+     */
     DTMF_SUPPORT_CHANGED: "conference.dtmf_support_changed"
 };
 

doc/API.md

         - IN_LAST_N_CHANGED - passes boolean property that shows whether the local user is included in last n set of any other user or not. (parameters - boolean)
         - CONFERENCE_JOINED - notifies the local user that he joined the conference successfully. (no parameters)
         - CONFERENCE_LEFT - notifies the local user that he left the conference successfully. (no parameters)
+        - DTMF_SUPPORT_CHANGED - notifies if at least one user supports DTMF. (parameters - supports(boolean))
 
     2. connection
         - CONNECTION_FAILED - indicates that the server connection failed.
 
 17. addTrack(track) - Adds JitsiLocalTrack object to the conference.
     - track - the JitsiLocalTrack
+
 18. removeTrack(track) - Removes JitsiLocalTrack object to the conference.
     - track - the JitsiLocalTrack
 
+19. isDTMFSupported() - Check if at least one user supports DTMF.
 
 JitsiTrack
 ======