bpo-31904: Add posix module support for VxWorks (GH-12118)

 
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index e77a8fe..0bbfce9 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -32,6 +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.
 
 .. note::
 
@@ -3578,6 +3579,9 @@
    process.  On Windows, the process id will actually be the process handle, so can
    be used with the :func:`waitpid` function.
 
+   Note on VxWorks, this function doesn't return ``-signal`` when the new process is
+   killed. Instead it raises OSError exception.
+
    The "l" and "v" variants of the :func:`spawn\* <spawnl>` functions differ in how
    command-line arguments are passed.  The "l" variants are perhaps the easiest
    to work with if the number of parameters is fixed when the code is written; the