Issue #13883: Document all platforms PYTHONCASEOK works on.
diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
index fed83df..29d249f 100644
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -512,7 +512,7 @@
 .. envvar:: PYTHONCASEOK
 
    If this is set, Python ignores case in :keyword:`import` statements.  This
-   only works on Windows.
+   only works on Windows, OS X, OS/2, and RiscOS.
 
 
 .. envvar:: PYTHONDONTWRITEBYTECODE