mention graph vis isn't available in Release builds.

llvm-svn: 132916
diff --git a/llvm/docs/ProgrammersManual.html b/llvm/docs/ProgrammersManual.html
index 060bbb6..3bec325 100644
--- a/llvm/docs/ProgrammersManual.html
+++ b/llvm/docs/ProgrammersManual.html
@@ -811,6 +811,10 @@
 Attributes</a>.)  If you want to restart and clear all the current graph
 attributes, then you can <tt>call DAG.clearGraphAttrs()</tt>. </p>
 
+<p>Note that graph visualization features are compiled out of Release builds
+to reduce file size.  This means that you need a Debug+Asserts or 
+Release+Asserts build to use these features.</p>
+
 </div>
 
 </div>