[PATCH] Add note to documentation about machine node chains.

[hfinkel at anl.gov]

LGTM.

================
Comment at: docs/CodeGenerator.rst:774
@@ -773,1 +773,3 @@
+machine nodes have their chain after the instruction's operands, and
+by be followed by glue nodes.
 
----------------
by be -> maybe

http://reviews.llvm.org/D4698