message: Add missing Put signature
diff: @@ -84,6 +84,13 @@ The following Variable<T>::Span signatures are available:
  Variable<T>::Span Put(Variable<T> variable);
  +3. Return a span setting an initial fill value into a certain buffer. If span is not returned then the fillValue is fixed for that block.
  +
  + .. code-block:: c++
  +
  + Variable<T>::Span Put(Variable<T> variable, const size_t bufferID, const T fillValue);
  +
  +
  .. warning::
  As of version 2.4.0 only the default BP3 engine using the C++11 bindings supports ``Variable<T>::Span`` Put signatures. We plan to support this feature and add this to streaming Engines.
  