Help: Add version information for SYSTEM option of add_subdirectory

Help/command/add_subdirectory.rst

@@ -34,9 +34,10 @@ parent project depends on a target in the subdirectory, the dependee
 target will be included in the parent project build system to satisfy
 the dependency.
 
-If the ``SYSTEM`` argument is provided, the :prop_dir:`SYSTEM` directory
-property of the subdirectory will be set to true.  This property is
-used to initialize the :prop_tgt:`SYSTEM` property of each non-imported
-target created in that subdirectory.  The include directories of targets
-with :prop_tgt:`SYSTEM` set to true will be treated as ``SYSTEM`` when
-compiling consumers.
+.. versionadded:: 3.25
+  If the ``SYSTEM`` argument is provided, the :prop_dir:`SYSTEM` directory
+  property of the subdirectory will be set to true.  This property is
+  used to initialize the :prop_tgt:`SYSTEM` property of each non-imported
+  target created in that subdirectory.  The include directories of targets
+  with :prop_tgt:`SYSTEM` set to true will be treated as ``SYSTEM`` when
+  compiling consumers.