bpo-31904: Don't build the _crypt extension on VxWorks (GH-12833)

diff --git a/Doc/library/crypt.rst b/Doc/library/crypt.rst
index 43d4b5b..d25c626 100644
--- a/Doc/library/crypt.rst
+++ b/Doc/library/crypt.rst
@@ -30,6 +30,8 @@
 extensions available on the current implementation will also  be available on
 this module.
 
+.. availability:: Unix. Not available on VxWorks.
+
 Hashing Methods
 ---------------