Correct the documentation for crs_matrix.h

Thanks to Joydeep Biswas for reporting this.

Change-Id: Iae5fc2274644aab40f2f922a671f65da15ae71fc

include/ceres/crs_matrix.h

diff --git a/include/ceres/crs_matrix.h b/include/ceres/crs_matrix.h
index c9fe8f7..8c470cd 100644
--- a/include/ceres/crs_matrix.h
+++ b/include/ceres/crs_matrix.h
@@ -44,17 +44,35 @@
   int num_rows;
   int num_cols;
 
-  // A compressed row matrix stores its contents in three arrays.
-  // The non-zero pattern of the i^th row is given by
+  // A compressed row matrix stores its contents in three arrays,
+  // rows, cols and values.
   //
-  //   rows[cols[i] ... cols[i + 1]]
+  // rows is a num_rows + 1 sized array that points into the cols and
+  // values array. For each row i:
   //
-  // and the corresponding values by
+  // cols[rows[i]] ... cols[rows[i + 1] - 1] are the indices of the
+  // non-zero columns of row i.
   //
-  //   values[cols[i] ... cols[i + 1]]
+  // values[rows[i]] .. values[rows[i + 1] - 1] are the values of the
+  // corresponding entries.
   //
-  // Thus, cols is a vector of size num_cols + 1, and rows and values
-  // have as many entries as number of non-zeros in the matrix.
+  // cols and values contain as many entries as there are non-zeros in
+  // the matrix.
+  //
+  // e.g, consider the 3x4 sparse matrix
+  //
+  //  [ 0 10  0  4 ]
+  //  [ 0  2 -3  2 ]
+  //  [ 1  2  0  0 ]
+  //
+  // The three arrays will be:
+  //
+  //
+  //            -row0-  ---row1---  -row2-
+  //  rows   = [ 0,      2,          5,     7]
+  //  cols   = [ 1,  3,  1,  2,  3,  0,  1]
+  //  values = [10,  4,  2, -3,  2,  1,  2]
+
   vector<int> cols;
   vector<int> rows;
   vector<double> values;