Help: Document MANUALLY_ADDED_DEPENDENCIES in add_dependencies

This adds a note about the MANUALLY_ADDED_DEPENDENCIES target property
to the add_dependencies documentation page. It was introduced in v3.8.0.

Help/command/add_dependencies.rst

@@ -20,6 +20,10 @@ transitively in its place since the target itself does not build.
 .. versionadded:: 3.3
   Allow adding dependencies to interface libraries.
 
+.. versionadded:: 3.8
+  Dependencies will populate the :prop_tgt:`MANUALLY_ADDED_DEPENDENCIES`
+  property of ``<target>``.
+
 .. versionchanged:: 3.9
   The :ref:`Ninja Generators` use weaker ordering than
   other generators in order to improve available concurrency.