gh-44123: Add note on relative path for os.exec* (GH-93826)

Co-authored-by: Steve Dower <[email protected]>

Author: slateny

Doc/library/os.rst

@@ -3919,7 +3919,8 @@ to be ignored.
    the :envvar:`PATH` variable. The other variants, :func:`execl`, :func:`execle`,
    :func:`execv`, and :func:`execve`, will not use the :envvar:`PATH` variable to
    locate the executable; *path* must contain an appropriate absolute or relative
-   path.
+   path. Relative paths must include at least one slash, even on Windows, as
+   plain names will not be resolved.
 
    For :func:`execle`, :func:`execlpe`, :func:`execve`, and :func:`execvpe` (note
    that these all end in "e"), the *env* parameter must be a mapping which is