message: Add documentation to `FABilinearFormExtension::EliminateBC`.
diff: @@ -126,6 +126,8 @@ public:
  void Assemble();
  void RAP(OperatorHandle &A);
  + /** @note Always does `DIAG_ONE` policy to be consistent with
  + `ConstrainedOperator`. */
  void EliminateBC(const Array<int> &ess_dofs, OperatorHandle &A);
  void FormSystemMatrix(const Array<int> &ess_tdof_list, OperatorHandle &A);
  void FormLinearSystem(const Array<int> &ess_tdof_list,
  