fix doc comment bug, noticed by Jochen

llvm-svn: 129186

llvm/include/llvm/Support/Casting.h

@@ -192,8 +192,8 @@ template<class To, class FromTy> struct cast_convert_val<To,FromTy,FromTy> {
 
 // cast<X> - Return the argument parameter cast to the specified type.  This
 // casting operator asserts that the type is correct, so it does not return null
-// on failure.  But it will correctly return NULL when the input is NULL.
-// Used Like this:
+// on failure.  It does not allow a null argument (use cast_or_null for that).
+// It is typically used like this:
 //
 //  cast<Instruction>(myVal)->getParent()
 //