I am using the following request to create a poll through the REST API.
I am obviously missing something, since my error is Unrecognized field "choices"
I am doing this via the REST Explorer in the workbench.

{ 
   "feedElementType":"FeedItem",
   "subjectId":"00570000001J64WAAS",
   "body" : {
      "messageSegments" : [      {
         "type" : "Text",
         "text" : "When should we meet for UDW planning?"}]
   },
   "capabilities":{ 
   "choices" : [
      "Monday", "Tuesday"
   ]
   }
}