gh-37272: enable the generation of acyclic orientations with `nauty_directg`
Since version 2.8, nauty's `directg` has a new option `-a` to generate
only acyclic orientations. This option was already working but not
documented. This PR documents this functionality.
This partially answer issue #37253 and is related to PR #35075.
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URL: https://github.com/sagemath/sage/pull/37272
Reported by: David Coudert
Reviewer(s): Martin Rubey
