Add javadoc to Subsegment.close() method.

This method should not be used to end subsegments for segments using multiple threads as this can result in a deadlock. See #318 for details.

Author: Kurru

aws-xray-recorder-sdk-core/src/main/java/com/amazonaws/xray/entities/Subsegment.java

@@ -100,6 +100,12 @@ static Subsegment noOp(Segment parent, AWSXRayRecorder recorder) {
      */
     String prettyStreamSerialize();
 
+    /**
+     * Implements the {@link AutoCloseable} interface.
+     *
+     * <p>Warning. Avoid using this to end subsegments in asynchronous methods or other threads as this can cause
+     * deadlocks. Instead, use {@link com.amazonaws.xray.AWSXRay#endSubsegment(Subsegment)}.
+     */
     @Override
     void close();
 }