gh-101100: Fix broken xrefs in fcntl module doc (#115691)

* 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

Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>

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Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>