Doc: improve documentation of Map<SparseMatrix>

doc/SparseQuickReference.dox

@@ -206,7 +206,7 @@ See \ref TutorialSparse_SubMatrices and below for read-write sub-matrices.
   sm1.innerVectors(start, size);    // RW
   sm1.leftCols(size);               // RW
   sm2.rightCols(size);              // RO because sm2 is row-major
-  sm1.middleRows(start, numRows);   // RO becasue sm1 is column-major
+  sm1.middleRows(start, numRows);   // RO because sm1 is column-major
   sm1.middleCols(start, numCols);   // RW
   sm1.col(j);                       // RW
 \endcode
@@ -253,6 +253,20 @@ If the matrix is not in compressed form, makeCompressed() should be called befor
 Note that these functions are mostly provided for interoperability purposes with external libraries.\n
 A better access to the values of the matrix is done by using the InnerIterator class as described in \link TutorialSparse the Tutorial Sparse \endlink section</td>
 </tr>
+<tr class="alt"><td colspan="2">Mapping external buffers</td></tr>
+<tr class="alt">
+<td>
+\code
+int outerIndexPtr[cols+1];
+int innerIndices[nnz];
+double values[nnz];
+Map<SparseMatrix<double> > sm1(rows,cols,nnz,outerIndexPtr, // read-write
+                               innerIndices,values);
+Map<const SparseMatrix<double> > sm2(...);                  // read-only
+\endcode
+</td>
+<td>As for dense matrices, class Map<SparseMatrixType> can be used to see external buffers as an %Eigen's SparseMatrix object. </td>
+</tr>
 </table>
 */
 }