[Docu] Fix doxygen warnings.

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 domain.

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. The parameter have to defined on the line domain.
-
-The user should carefully check which physical quantity that parameter is.

Documentation/ProjectFile/prj/process_variables/process_variable/source_terms/source_term/Volumetric/c_Volumetric.md

-Declares a source term that is defined on the entire domain.
+Declares a source term that is defined on a sub-domain (volumetric) with the
+same dimensionality d or on a 1d (line mesh) sub-domain.
 
 The user should carefully check which physical quantity that parameter is.

ProcessLib/SourceTerms/CreateVolumetricSourceTerm.cpp

@@ -30,11 +30,13 @@ std::unique_ptr<SourceTerm> createVolumetricSourceTerm(
     auto const type = config.peekConfigParameter<std::string>("type");
     if (type == "Line")
     {
+        //! \ogs_file_param{prj__process_variables__process_variable__source_terms__source_term__type}
         config.checkConfigParameter("type", "Line");
         DBUG("Constructing LineSourceTerm from config.");
     }
     else
     {
+        //! \ogs_file_param{prj__process_variables__process_variable__source_terms__source_term__type}
         config.checkConfigParameter("type", "Volumetric");
         DBUG("Constructing VolumetricSourceTerm from config.");
     }