added extra BUFFERSIZE doc and getter

Conflicts:
	modules/highgui/doc/reading_and_writing_images_and_video.rst
	modules/highgui/src/cap_dc1394_v2.cpp

modules/videoio/src/cap_dc1394_v2.cpp

@@ -689,6 +689,8 @@ double CvCaptureCAM_DC1394_v2_CPP::getProperty(int propId) const
         break;
     case CV_CAP_PROP_ISO_SPEED:
         return (double) isoSpeed;
+    case CV_CAP_PROP_BUFFERSIZE:
+        return (double) nDMABufs;
     default:
         if (propId<CV_CAP_PROP_MAX_DC1394 && dc1394properties[propId]!=-1
             && dcCam)