clarified docs

docs/docs/imaging.xml

@@ -325,7 +325,7 @@
          <description>
                 This object is a tool for performing image feature extraction.  In
                 particular, it wraps another image feature extractor and converts
-                the wrapped image feature vectors into indicator vectors.  It does
+                the wrapped image feature vectors into sparse indicator vectors.  It does
                 this by hashing each feature vector and then returns a new vector 
                 which is zero everywhere except for the position determined by the 
                 hash.