bpo-38378: Rename parameters "out" and "in" of os.sendfile(). (GH-16742)

They conflicted with keyword "in".

Also rename positional-only parameters of private os._fcopyfile()
for consistency.
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 67fe36b..fe9531b 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -1251,27 +1251,27 @@
       :exc:`InterruptedError` exception (see :pep:`475` for the rationale).
 
 
-.. function:: sendfile(out, in, offset, count)
-              sendfile(out, in, offset, count, [headers], [trailers], flags=0)
+.. function:: sendfile(out_fd, in_fd, offset, count)
+              sendfile(out_fd, in_fd, offset, count, [headers], [trailers], flags=0)
 
-   Copy *count* bytes from file descriptor *in* to file descriptor *out*
+   Copy *count* bytes from file descriptor *in_fd* to file descriptor *out_fd*
    starting at *offset*.
-   Return the number of bytes sent. When EOF is reached return 0.
+   Return the number of bytes sent. When EOF is reached return ``0``.
 
    The first function notation is supported by all platforms that define
    :func:`sendfile`.
 
    On Linux, if *offset* is given as ``None``, the bytes are read from the
-   current position of *in* and the position of *in* is updated.
+   current position of *in_fd* and the position of *in_fd* is updated.
 
    The second case may be used on Mac OS X and FreeBSD where *headers* and
    *trailers* are arbitrary sequences of buffers that are written before and
-   after the data from *in* is written. It returns the same as the first case.
+   after the data from *in_fd* is written. It returns the same as the first case.
 
-   On Mac OS X and FreeBSD, a value of 0 for *count* specifies to send until
-   the end of *in* is reached.
+   On Mac OS X and FreeBSD, a value of ``0`` for *count* specifies to send until
+   the end of *in_fd* is reached.
 
-   All platforms support sockets as *out* file descriptor, and some platforms
+   All platforms support sockets as *out_fd* file descriptor, and some platforms
    allow other types (e.g. regular file, pipe) as well.
 
    Cross-platform applications should not use *headers*, *trailers* and *flags*
@@ -1286,6 +1286,9 @@
 
    .. versionadded:: 3.3
 
+   .. versionchanged:: 3.9
+      Parameters *out* and *in* was renamed to *out_fd* and *in_fd*.
+
 
 .. function:: set_blocking(fd, blocking)