improve tblgen doc, pointed out by Zhongxing.

llvm-svn: 102072

llvm/docs/TableGenFundamentals.html

@@ -333,8 +333,9 @@ The TableGen types are:</p>
   <dd>This type represents a nestable directed graph of elements.</dd>
 
 <dt><tt><b>code</b></tt></dt>
-  <dd>This represents a big hunk of text. NOTE: I don't remember why this is
-  distinct from string!</dd>
+  <dd>This represents a big hunk of text.  This is lexically distinct from 
+  string values because it doesn't require escapeing double quotes and other
+  common characters that occur in code.</dd>
 </dl>
 
 <p>To date, these types have been sufficient for describing things that