Update graphlib.py

[nikolasavic3]

docs: fix CycleError and prepare() documentation clarity

Improves clarity and accuracy in the documentation:

• Clarify that cycle selection is arbitrary in CycleError docstring
• Improve prepare() documentation to explicitly state call requirements
• Make prepare() error documentation more structured

[ghost]

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[picnixz]

Some changes are incorrect and are only cosmetic. I suspect this was generated by IA. I am therefore closing it. If you think there is an issue with the current doc, please open a separate issue first (also, the PR is modifying code and typos and I think its scope is incorrect).

[picnixz]

Update: The use of :exec: instead of exc is a typo which can be corrected, but please do so it a separate PR that only fixes typos (as well as a the "a topological order")