add_dependencies: Document behavior under Ninja generators

Closes: #25796

Help/command/add_dependencies.rst

@@ -20,6 +20,13 @@ transitively in its place since the target itself does not build.
 .. versionadded:: 3.3
   Allow adding dependencies to interface libraries.
 
+.. versionchanged:: 3.9
+  The :ref:`Ninja Generators` use weaker ordering than
+  other generators in order to improve available concurrency.
+  They only guarantee that the dependencies' custom commands are
+  finished before sources in ``<target>`` start compiling; this
+  ensures generated sources are available.
+
 See Also
 ^^^^^^^^