Help/OBJC_STANDARD: Document Objective C17 and C23 support

This was missed in

* commit 93b7d3d292 (C17 support, 2021-02-06, v3.21.0-rc1~587^2~10)
* commit dcd599757f (C23 support, 2021-02-06, v3.21.0-rc1~587^2~9)

Fixes: #25746

Help/prop_tgt/OBJC_STANDARD.rst

@@ -20,6 +20,16 @@ Supported values are:
 ``11``
   Objective C11
 
+``17``
+  .. versionadded:: 3.21
+
+  Objective C17
+
+``23``
+  .. versionadded:: 3.21
+
+  Objective C23
+
 If the value requested does not result in a compile flag being added for
 the compiler in use, a previous standard flag will be added instead.  This
 means that using: