Wrap XML in Doxygen comment with \code block

Fix Doxygen warnings about unknown XML tags and add syntax highlighting
to documentation using the Doxygen `\code{.xml}` and `\endcode` commands

Author: John Nonweiler

scripts/expected_doxygen_warnings.txt

@@ -155,16 +155,6 @@
   parameter 'clas_hierarchy'
 /cbmc/jbmc/src/java_bytecode/select_pointer_type.h:50: warning: The following parameters of select_pointer_typet::specialize_generics(const pointer_typet &pointer_type, const generic_parameter_specialization_mapt &generic_parameter_specialization_map, generic_parameter_recursion_trackingt &visited_nodes) const are not documented:
   parameter 'visited_nodes'
-/cbmc/src/goto-programs/show_goto_functions_xml.cpp:35: warning: Unsupported xml/html tag <functions> found
-/cbmc/src/goto-programs/show_goto_functions_xml.cpp:36: warning: Unsupported xml/html tag <function> found
-/cbmc/src/goto-programs/show_goto_functions_xml.cpp:37: warning: Unsupported xml/html tag <instructions> found
-/cbmc/src/goto-programs/show_goto_functions_xml.cpp:37: warning: Unsupported xml/html tag <location> found
-/cbmc/src/goto-programs/show_goto_functions_xml.cpp:38: warning: Unsupported xml/html tag <instruction_value> found
-/cbmc/src/goto-programs/show_goto_functions_xml.cpp:39: warning: Unsupported xml/html tag </instruction_value> found
-/cbmc/src/goto-programs/show_goto_functions_xml.cpp:39: warning: Unsupported xml/html tag </instruction> found
-/cbmc/src/goto-programs/show_goto_functions_xml.cpp:39: warning: Unsupported xml/html tag </instructions> found
-/cbmc/src/goto-programs/show_goto_functions_xml.cpp:39: warning: Unsupported xml/html tag </function> found
-/cbmc/src/goto-programs/show_goto_functions_xml.cpp:39: warning: Unsupported xml/html tag </functions> found
 /cbmc/src/solvers/refinement/string_refinement.cpp:2099: warning: argument 'expr' of command @param is not found in the argument list of string_constraintt::universal_only_in_index(const string_constraintt &constr)
 /cbmc/src/solvers/refinement/string_refinement.cpp:2106: warning: The following parameters of string_constraintt::universal_only_in_index(const string_constraintt &constr) are not documented:
   parameter 'constr'

src/goto-programs/show_goto_functions_xml.cpp

@@ -33,11 +33,13 @@ show_goto_functions_xmlt::show_goto_functions_xmlt(
 {}
 
 /// Walks through all of the functions in the program and returns an xml object
-/// representing all their functions. Produces output like this: <functions>
+/// representing all their functions. Produces output like this: \code{.xml}
+/// <functions>
 /// <function name=main, is_body_available=true, is_internal=false>
 /// <instructions> <instruction_id=ASSIGN> <location file="main.c" line="14"/>
 /// <instruction_value> // 34 file main.c line 1 s = { 'a', 'b', 'c', 0 };
-/// </instruction_value> </instruction> </instructions> </function> </functions>
+/// </instruction_value> </instruction> </instructions> </function>
+/// </functions> \endcode
 /// \param goto_functions: the goto functions that make up the program
 xmlt show_goto_functions_xmlt::convert(
   const goto_functionst &goto_functions)