add documentation of 3 parameters

fix typo errors

fix lint error

Extend docu for is_linear

fix lint error

add reference

fix reference error

fix doc error

doc of weighting factor

improve explanation

fix lint

Documentation/ProjectFile/prj/processes/process/HEAT_TRANSPORT_BHE/t_linear.md

+The default setting is false. When activated, non-linear iterations will be turned off. The simulation will just go through a single iteration within a time step. If it's set to true, the **\<use_algebraic_bc\>** option must be also set to true. Only when fluid properties does not change much with temperature, this feature can be activated. The effect of skipping the non-linear iteration is much faster simulation speed.

Documentation/ProjectFile/prj/processes/process/HEAT_TRANSPORT_BHE/t_use_algebraic_bc.md

+The default setting is false. When algebraic boundary condition is activated, the inflow temperatures at the top and bottom of the BHE are no longer imposed as a Dirichlet type boundary condition. Instead, the linear equation system will be enlarged to accommodate the numerical constrains. The effect of this feature is to accelerate BHE simulations.

Documentation/ProjectFile/prj/processes/process/HEAT_TRANSPORT_BHE/t_weighting_factor.md

+This value is the weighting factor used to impose an algebraic boundary condition in the **HeatTransportBHE** process. The default value is 100. This value is only effective, when the option **\<use_algebraic_bc\>** is set to true. This value is multiplied with the max value of the column wise inner product and then imposed in the additional rows at the end of the linear equation system, so that the unknown temperatures at the top and bottom of the BHE are imposed with certain desired values.