[PL] ConstraintDirichletBC: Docu for the website.

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

+For the constraint Dirichlet-type boundary condition the type has to be
+ConstraintDirichlet.

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

+This tag specifies the process variable whose flux values constrain the current
+process variable.

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

+The value of the tag determines the evaluation of the constraint condition.
+Possible values for the tag 'constraint_directions' are 'greater' or 'lower'.
+
+If the value 'greater' is given the condition 'calculated_flux_value >
+constraint_threshold' is evaluated. If the value 'less' is given the condition
+'calculated_flux_value < constraint_threshold' is evaluated.

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

+Threshold value used in the constraint condition.

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.

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

+The name of the parameter that defines the Dirichlet-type condition values.