Small addition to sparse doc.

doc/doc/sparse.txt

@@ -82,6 +82,10 @@ Several things should be learned from the above example:
 
 TODO: Rewrite this documentation to do things in a smarter way.
 
+Misc
+----------------------------------------
+The sparse equivalent of dmatrix is csc_matrix and csr_matrix.
+
 :api:`TrueDot` vs. :api:`StructuredDot`
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