Issue #13530: Document os.lseek() result

Patch written by Jérémy Anger.
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 767c40f..df9a6b6 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -706,7 +706,7 @@
    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.
 
diff --git a/Misc/ACKS b/Misc/ACKS
index 0f92165..24923da 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -30,6 +30,7 @@
 Erik Andersén
 Oliver Andrich
 Ross Andrus
+Jérémy Anger
 Jon Anglin
 Éric Araujo
 Alicia Arlen
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 66cd153..673877a 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -5687,7 +5687,8 @@
 
 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)