docs: downcasting documentation added

docs/CMakeLists.txt

@@ -119,6 +119,7 @@ add_custom_command(OUTPUT "${SPHINX_INDEX_FILE}"
                        "${CMAKE_CURRENT_SOURCE_DIR}/reference/core.rst"
                        "${CMAKE_CURRENT_SOURCE_DIR}/reference/core/concepts.rst"
                        "${CMAKE_CURRENT_SOURCE_DIR}/reference/core/customization_points.rst"
+                       "${CMAKE_CURRENT_SOURCE_DIR}/reference/core/downcasting.rst"
                     #    "${CMAKE_CURRENT_SOURCE_DIR}/reference/core/functions.rst"
                        "${CMAKE_CURRENT_SOURCE_DIR}/reference/core/metafunctions.rst"
                        "${CMAKE_CURRENT_SOURCE_DIR}/reference/core/types.rst"

docs/reference/core.rst

@@ -9,3 +9,4 @@ Core Library
     core/functions
     core/metafunctions
     core/customization_points
+    core/downcasting

docs/reference/core/downcasting.rst

@@ -0,0 +1,18 @@
+The Downcasting Facility
+========================
+
+.. concept:: template<typename T> Downcastable
+
+.. doxygenstruct:: units::downcast_base
+   :members:
+
+.. doxygenstruct:: units::downcast_child
+   :members:
+
+.. doxygenstruct:: units::downcast_poison
+   :members:
+
+.. doxygenstruct:: units::downcast_dispatch
+   :members:
+
+.. doxygentypedef:: units::downcast