Remove unnecessary mention of os.fsync() from os.exec*() docs

Before a process is replaced by os.exec*(), userspace buffers usually
need to be flushed. However, it is not necessary to flush kernel buffers
to disk using os.fsync(). Flushing userspace buffer to kernel, and
flushing kernel buffer to disk, are two different things. In addition,
os.fsync() is not applicable to pipes or sockets. This patch removes
this confusing information.

Author: zhangboyang

Doc/library/os.rst

@@ -4318,11 +4318,10 @@ to be ignored.
    and will have the same process id as the caller.  Errors will be reported as
    :exc:`OSError` exceptions.
 
-   The current process is replaced immediately. Open file objects and
-   descriptors are not flushed, so if there may be data buffered
-   on these open files, you should flush them using
-   :func:`sys.stdout.flush` or :func:`os.fsync` before calling an
-   :func:`exec\* <execl>` function.
+   The current process is replaced immediately. Open file objects are not
+   flushed, so if there may be data buffered on these open files, you should
+   flush them using :meth:`~io.IOBase.flush`, e.g. ``sys.stdout.flush()``,
+   before calling an :func:`exec\* <execl>` function.
 
    The "l" and "v" variants of the :func:`exec\* <execl>` functions differ in how
    command-line arguments are passed.  The "l" variants are perhaps the easiest