Added doc string.

theano/sparse/basic.py

@@ -618,6 +618,9 @@ class DenseFromSparse(gof.op.Op):
 dense_from_sparse = DenseFromSparse()
 
 class SparseFromDense(gof.op.Op):
+    """
+    Convert an `ndarray` matrix to a sparse matrix.
+    """
     def __init__(self, format):
         self.format = format
     def __eq__(self, other):