[3.12] gh-101100: Fix broken xrefs in fcntl module doc (GH-115691) (#115924)

* clean up fcntl module doc

* simplify

* a few changes, based on suggestion by CAM-Gerlach

* nitpick ignore for a couple other C functions mentioned in the fcntl module doc

* more changes, especially related to LOCK_* constants

* :data: back to :const:

* Apply suggestions from code review



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(cherry picked from commit 84a275c4a2c8a22d198c6f227d538e6b27bbb029)

Co-authored-by: Skip Montanaro <skip.montanaro@gmail.com>
Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>