Copy documentation from nonuniform Neumann.

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/NonuniformDirichlet/c_NonuniformDirichlet.md

+Declares a Dirichlet boundary condition that is non-uniform in space.
+The values are read from the given boundary mesh.

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/NonuniformDirichlet/t_field_name.md

+This specifies a nodal field defined on the surface mesh (see the \c mesh
+parameter of the nonuniform Dirichlet BC) and the values used for the boundary
+condition.

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/NonuniformDirichlet/t_mesh.md

+Path to the surface mesh vtu file.
+
+The surface mesh must contain a nodal integer-valued field (unsigned 64bit)
+named \c OriginalSubsurfaceNodeIDs. That field establishes the mapping between
+the nodes of the surface mesh to some notes in the bulk mesh.
+\warning It is not checked if the surface mesh and the bulk mesh correspond to each
+other; in particular it is not checked if surface and bulk nodes coincide and if
+surface elements coincide with the faces of bulk elements.