Updated the boostbook.dtd and reference docs

for "enumvalue" elements to indicate they can have "purpose" 
and "description" children.


[SVN r42389]

tools/boostbook

@@ -1 +1 @@
-Subproject commit 75f209265ca93a3c700056b5d806c46f0381697a
+Subproject commit 4017a76615b0b80d4fa759b2e9c47b14304fad49