bpo-41224: Add versionadded for Symbol.is_annotated (GH-23861)

diff --git a/Doc/library/symtable.rst b/Doc/library/symtable.rst
index 40c1795..e364232 100644
--- a/Doc/library/symtable.rst
+++ b/Doc/library/symtable.rst
@@ -160,6 +160,8 @@
 
       Return ``True`` if the symbol is annotated.
 
+      .. versionadded:: 3.6
+
    .. method:: is_free()
 
       Return ``True`` if the symbol is referenced in its block, but not assigned