Doc: fcntl.lockf() is more powerful than written (GH-6750)

Doc/library/fcntl.rst

@@ -124,7 +124,8 @@ The module defines the following functions:
 .. function:: lockf(fd, cmd, len=0, start=0, whence=0)
 
    This is essentially a wrapper around the :func:`~fcntl.fcntl` locking calls.
-   *fd* is the file descriptor of the file to lock or unlock, and *cmd*
+   *fd* is the file descriptor (file objects providing a :meth:`~io.IOBase.fileno`
+   method are accepted as well) of the file to lock or unlock, and *cmd*
    is one of the following values:
 
    * :const:`LOCK_UN` -- unlock
@@ -175,4 +176,3 @@ using the :func:`flock` call may be better.
       present in the :mod:`os` module (on BSD only), the :func:`os.open`
       function provides an alternative to the :func:`lockf` and :func:`flock`
       functions.
-