Add note to documentation about machine node chains.

I've been assuming chain operands were always the first operand,
since the documentation says this. I was confused about why they
were missing after instruction selection. Apparently the convention
changes to using the last operand for MachineSDNodes and I've never
noticed before.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216934 91177308-0d34-0410-b5e6-96231b3b80d8