Revert "[doc] Document thread safety guarantees of MatrixIndexSet"

This reverts commit aa9408f1.

dune/istl/matrixindexset.hh

@@ -28,10 +28,6 @@ namespace Dune {
    * The default value for maxVectorSize works well and ensures
    * that the slow std::set fallback is only used for very
    * dense rows.
-   *
-   * This class is thread safe in the sense that concurrent calls
-   * to all const methods and furthermore to add(row,col) with different
-   * rows in each thread are safe.
    */
   class MatrixIndexSet
   {
@@ -102,13 +98,7 @@ namespace Dune {
       indices_.resize(rows_, FlatSet());
     }
 
-    /**
-     * \brief Add an index to the index set
-     *
-     * It is safe to call add(row, col) for different rows in concurrent threads,
-     * but it is not safe to do concurrent calls for the same row, even for different
-     * columns.
-     */
+    /** \brief Add an index to the index set */
     void add(size_type row, size_type col) {
       return std::visit(Dune::overload(
         // If row is stored as set, call insert directly