docs: mention the ongoing work on shifting algorithm in the API notat…

…ion (#747)

* docs: mention the ongoing work on shifting algorithm in the API notation, when discussing flex models

Signed-off-by: Nicolas Höning <nicolas@seita.nl>

* Add hint about using storage-efficiency in buffer scheduling

Signed-off-by: Nicolas Höning <nicolas@seita.nl>

---------

Signed-off-by: Nicolas Höning <nicolas@seita.nl>

documentation/api/notation.rst

@@ -204,13 +204,14 @@ Here are the three types of flexibility models you can expect to be built-in:
 
 2) For **shiftable processes**
 
-   .. todo:: A simple algorithm exists, needs integration into FlexMeasures and asset type clarified.
+   .. todo:: A simple and proven algorithm exists, but is awaiting proper integration into FlexMeasures, see `PR 729 <https://github.com/FlexMeasures/flexmeasures/pull/729>`_.
 
 3) For **buffer devices** (e.g. thermal energy storage systems connected to heat pumps), use the same flexibility parameters described above for storage devices. Here are some tips to model a buffer with these parameters:
 
    - Describe the thermal energy content in kWh or MWh.
    - Set ``soc-minima`` to the accumulative usage forecast.
    - Set ``roundtrip-efficiency`` to the square of the conversion efficiency. [#]_
+   - Set ``storage-efficiency`` to a value below 100% to model (heat) loss.
 
     .. [#] Setting a roundtrip efficiency of higher than 1 is not supported. We plan to implement a separate field for :abbr:`COP (coefficient of performance)` values.