COSMIT fix a docstring in feature_extraction.text

I amended aeeda9f to get the tests to
pass, but forgot to change this one.

Author: larsmans

sklearn/feature_extraction/text.py

@@ -441,10 +441,7 @@ def _get_hasher(self):
 
 
 def _document_frequency(X):
-    """Count the number of non-zero values for each feature in X.
-
-    If X is sparse, its data must not contain explicit zero values.
-    """
+    """Count the number of non-zero values for each feature in csc_matrix X."""
     return np.diff(X.indptr)