[docu] Line source term docu.

Documentation/ProjectFile/prj/process_variables/process_variable/source_terms/source_term/Line/c_Line.md

+Declares a source term that is defined on a line shaped sub-domain (line mesh).
+
+The user should carefully check which physical quantity that parameter is.

Documentation/ProjectFile/prj/process_variables/process_variable/source_terms/source_term/Line/t_parameter.md

+The name of the parameter that defines the value that should be used for the source
+term. Nodal parameters are not supported.
+
+The user should carefully check which physical quantity that parameter is.