Clarify cursor positioning

docs/api-spec-oai/nakadi-oai-0.6.0.yaml

@@ -399,9 +399,10 @@ paths:
 
         The event stream is formatted as a sequence of `EventStreamBatch`es separated by `\n`. Each
         `EventStreamBatch` contains a chunk of Events and a `Cursor` pointing to the **end** of the
-        chunk (i.e. last delivered Event). The cursor might specify the offset with the symbolic
-        value `BEGIN`, which will open the stream starting from the oldest available offset in the
-        partition.
+        chunk (i.e. last delivered Event). The default cursor position is at the front of the 
+        stream, with new events that arrive being sent to the client. The cursor's position in the 
+        stream can be specified by sending the value `BEGIN`, which opens the stream starting from 
+        the oldest available offset in the partition. 
 
         Currently the `application/x-json-stream` format is the only one supported by the system,
         but in the future other media types may be supported.