bpo-31904: Disable os.popen and popen test cases on VxWorks (GH-21687)

diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index a4c5fbb..b309988 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -32,7 +32,7 @@
   objects, and result in an object of the same type, if a path or file name is
   returned.
 
-* On VxWorks, os.fork, os.execv and os.spawn*p* are not supported.
+* On VxWorks, os.popen, os.fork, os.execv and os.spawn*p* are not supported.
 
 .. note::