Add iiasa#808 to docs, release notes

Author: khaeru

RELEASE_NOTES.rst

@@ -14,6 +14,7 @@ All changes
 - Support for Python 3.8 is dropped (:pull:`881`), as it has reached end-of-life.
 - Add :meth:`.Reporter.add_sankey` and :mod:`.tools.sankey` to create Sankey diagrams from solved scenarios (:pull:`770`).
   The :file:`westeros_sankey.ipynb` :ref:`tutorial <tutorial-westeros>` shows how to use this feature.
+- Add the :py:`concurrent=...` model option to :class:`.MACRO` (:pull:`808`).
 - Add option to :func:`.util.copy_model` from a non-default location of model files (:pull:`877`).
 
 .. _v3.9.0:

doc/api.rst

@@ -260,6 +260,14 @@ Model classes
    :exclude-members: items
    :show-inheritance:
 
+   The MACRO class solves only the MACRO model in “standalone” mode—that is, without MESSAGE.
+   It is also invoked from :class:`.MESSAGE_MACRO` to process *model_options* to control the behaviour of MACRO:
+
+   - **concurrent** (:class:`int` or :class:`float`, either :py:`0` or :py:`1`).
+     This corresponds to the GAMS compile-time variable ``MACRO_CONCURRENT``.
+     If set to :py:`0` (the default), MACRO is solved in a loop, once for each node in the Scenario.
+     If set to :py:`1`, MACRO is solved only once, for all nodes simultaneously.
+
    .. autoattribute:: items
       :no-value: