Issue python#13530: Document os.lseek() result

Patch written by Jérémy Anger.

Doc/library/os.rst

@@ -706,7 +706,7 @@ as internal buffering of data.
    by *how*: :const:`SEEK_SET` or ``0`` to set the position relative to the
    beginning of the file; :const:`SEEK_CUR` or ``1`` to set it relative to the
    current position; :const:`os.SEEK_END` or ``2`` to set it relative to the end of
-   the file.
+   the file. Return the new cursor position in bytes, starting from the beginning.
 
    Availability: Unix, Windows.
 

Misc/ACKS

@@ -30,6 +30,7 @@ John Anderson
 Erik Andersén
 Oliver Andrich
 Ross Andrus
+Jérémy Anger
 Jon Anglin
 Éric Araujo
 Alicia Arlen

Modules/posixmodule.c

@@ -5687,7 +5687,8 @@ posix_dup2(PyObject *self, PyObject *args)
 
 PyDoc_STRVAR(posix_lseek__doc__,
 "lseek(fd, pos, how) -> newpos\n\n\
-Set the current position of a file descriptor.");
+Set the current position of a file descriptor.\n\
+Return the new cursor position in bytes, starting from the beginning.");
 
 static PyObject *
 posix_lseek(PyObject *self, PyObject *args)