[docs] Improve cross-references in fcntl documentation (issue 19194)

[georg at python.org]

http://bugs.python.org/review/19194/diff/9474/Doc/library/fcntl.rst
File Doc/library/fcntl.rst (right):

http://bugs.python.org/review/19194/diff/9474/Doc/library/fcntl.rst#newcode1
Doc/library/fcntl.rst:1: :mod:`fcntl` --- The :c:func:`fcntl` and
:c:func:`ioctl` system calls
these references should just be replaced by ``fcntl`` and ``ioctl``.

http://bugs.python.org/review/19194/diff/9474/Doc/library/fcntl.rst#newcode128
Doc/library/fcntl.rst:128: * :const:`0` -- relative to the start of the
file (:const:`os.SEEK_SET`)
:data: is the correct role for module-level constants.

http://bugs.python.org/review/19194/