As requested, a blurb on sub-targets.

llvm-svn: 23769
diff --git a/llvm/docs/CodeGenerator.html b/llvm/docs/CodeGenerator.html
index 39eecc1..0e63fac 100644
--- a/llvm/docs/CodeGenerator.html
+++ b/llvm/docs/CodeGenerator.html
@@ -444,7 +444,11 @@
 
 <div class="doc_text">
   <p>
-  TODO
+  <p>The <tt>TargetSubtarget</tt> class is used to provide information about the
+  specific chip set being targeted.  A sub-target informs code generation of 
+  which instructions are supported, instruction latencies and instruction 
+  execution itinerary; i.e., which processing units are used, in what order, and
+  for how long.
   </p>
 </div>