Improve documentation.

diff --git a/include/deal.II/lac/petsc_block_sparse_matrix.h b/include/deal.II/lac/petsc_block_sparse_matrix.h
index 626974e70a0e2775ecf722dae72ba61bcabae3ad..15db6f4f317f95a9644710b537d2f4a8581c06e8 100644 (file)
--- a/include/deal.II/lac/petsc_block_sparse_matrix.h
+++ b/include/deal.II/lac/petsc_block_sparse_matrix.h
@@ -296,6 +296,11 @@ namespace PETScWrappers
 
       /**
        * This method assigns the PETSc Mat to the instance of the class.
+       *
+       * Note that the matrix is not copied: instead, the instance of this class
+       * is initialized to use the given matrix. This is useful if you want to
+       * interpret a PETSc Mat object as a deal.II BlockMatrix, and you already
+       * have a BlockMatrix object that you want to use for this purpose.
        */
       void
       assign_petsc_matrix(Mat A);