[docu] Update docu for constraint Dirichlet bc.

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/ConstraintDirichletBoundaryCondition/t_constraint_type.md

 The constraint type has to be 'Flux', i.e., the constraint is based on the
-secondary variable. It is planned to add constraints based on the value of the
-primary variables.
+secondary variable.
+
+Remark: There is also a Dirichlet-type boundary condition based on the primary
+variable.

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/PrimaryVariableConstraintDirichletBoundaryCondition/c_PrimaryVariableConstraintDirichletBoundaryCondition.md

 \copydoc ProcessLib::PrimaryVariableConstraintDirichletBoundaryCondition
+
+Remark: The is also a Dirichlet-type boundary condition based on the flux.

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/PrimaryVariableConstraintDirichletBoundaryCondition/t_constraint_direction.md → Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/PrimaryVariableConstraintDirichletBoundaryCondition/t_comparison_operator.md

-The tag 'constraint_direction' determines the evaluation of the constraint
+The tag 'comparison_operator' determines the evaluation of the constraint
 condition. Possible values are 'greater' or 'less'.
 
 \copydoc ProcessLib::PrimaryVariableConstraintDirichletBoundaryCondition::_less

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/PrimaryVariableConstraintDirichletBoundaryCondition/t_constraint_type.md

-The constraint type has to be 'PrimaryVariableConstraintDirichlet'. This enables
-constraints for the primary variable of the process.
-
-Remark: The is also a Dirichlet-type boundary condition based on the flux.

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/PrimaryVariableConstraintDirichletBoundaryCondition/t_constraint_threshold_parameter.md → Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/PrimaryVariableConstraintDirichletBoundaryCondition/t_threshold_parameter.md

-\copydoc ProcessLib::PrimaryVariableConstraintDirichletBoundaryCondition::_constraint_threshold_parameter
+\copydoc ProcessLib::PrimaryVariableConstraintDirichletBoundaryCondition::_threshold_parameter