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]

tools/boostbook

@@ -1 +1 @@
-Subproject commit 9d06dbbaae1f43bab2820ea07f53eda258a34aa7
+Subproject commit 38017203f69cec3aa5f402fe8306eb1deff67b9a