#6586: fix return/argument type doc for os.read() and os.write().

Doc/library/os.rst

@@ -540,9 +540,9 @@ by file descriptors.
 
 .. function:: read(fd, n)
 
-   Read at most *n* bytes from file descriptor *fd*. Return a string containing the
+   Read at most *n* bytes from file descriptor *fd*. Return a bytestring containing the
    bytes read.  If the end of the file referred to by *fd* has been reached, an
-   empty string is returned. Availability: Unix, Windows.
+   empty bytes object is returned. Availability: Unix, Windows.
 
    .. note::
 
@@ -574,8 +574,8 @@ by file descriptors.
 
 .. function:: write(fd, str)
 
-   Write the string *str* to file descriptor *fd*. Return the number of bytes
-   actually written. Availability: Unix, Windows.
+   Write the bytestring in *str* to file descriptor *fd*. Return the number of
+   bytes actually written. Availability: Unix, Windows.
 
    .. note::