#837577: note cryptic return value of spawn*e on invalid env dicts.

Doc/library/os.rst

@@ -1753,7 +1753,9 @@ written in Python, such as a mail server's external command delivery program.
    which is used to define the environment variables for the new process (they are
    used instead of the current process' environment); the functions
    :func:`spawnl`, :func:`spawnlp`, :func:`spawnv`, and :func:`spawnvp` all cause
-   the new process to inherit the environment of the current process.
+   the new process to inherit the environment of the current process.  Note that
+   keys and values in the *env* dictionary must be strings; invalid keys or
+   values will cause the function to fail, with a return value of ``127``.
 
    As an example, the following calls to :func:`spawnlp` and :func:`spawnvpe` are
    equivalent::