update the docs for cvExtractSURF and initModule_nonfree()

diff --git a/modules/nonfree/doc/feature_detection.rst b/modules/nonfree/doc/feature_detection.rst
index bd963fd..5d25c1e 100644 (file)
--- a/modules/nonfree/doc/feature_detection.rst
+++ b/modules/nonfree/doc/feature_detection.rst
@@ -126,3 +126,5 @@ Detects keypoints and computes SURF descriptors for them.
     :param params: SURF algorithm parameters in OpenCV 1.x API.
 
 The function is parallelized with the TBB library.
+
+If you are using the C version, make sure you call ``cv::initModule_nonfree()`` from ``nonfree/nonfree.hpp``.