message: Clarify OpDef.is_stateful flag definition
diff: @@ -126,6 +126,12 @@ message OpDef {
  // -------------------------------------------------------------------------
  // Optimization constraints.
  + // Ops are marked as stateful if their behavior depends on some state beyond
  + // their input tensors (e.g. variable reading op) or if they have
  + // a side-effect (e.g. printing or asserting ops). Equivalently, stateless ops
  + // must always produce the same output for the same input and have
  + // no side-effects.
  + //
  // By default Ops may be moved between devices. Stateful ops should
  // either not be moved, or should only be moved if that state can also
  // be moved (e.g. via some sort of save / restore).
  