Add a purpose for macroname, headername, enumname, globalname.

Ref #2153: There's still no documentation for methodname and classname.
These are actually defined in the docbook DTD for a different purpose so
I'm not sure how to deal with them.

http://www.docbook.org/tdg/en/html/methodname.html
http://www.docbook.org/tdg/en/html/classname.html

[SVN r51401]

doc/reference.dtdxml

@@ -1061,6 +1061,7 @@
     <pcdata/>
   </sequence-group>
 </content-model>
+<purpose>References a macro with the given name</purpose>
 </element>
 
 <attlist name="macroname">
@@ -1392,6 +1393,7 @@
     <pcdata/>
   </sequence-group>
 </content-model>
+<purpose>References a C++ header with the given name</purpose>
 </element>
 
 <attlist name="headername">
@@ -1788,6 +1790,7 @@
     <pcdata/>
   </sequence-group>
 </content-model>
+<purpose>References a global with the given name</purpose>
 </element>
 
 <attlist name="globalname">
@@ -3030,6 +3033,7 @@
     <pcdata/>
   </sequence-group>
 </content-model>
+<purpose>References an enumeration type with the given name</purpose>
 </element>
 
 <attlist name="enumname">