gh-38093: add doc on egf_to_ogf and inverse
Just adding a little comment in the doc on the link to Borel and Laplace
formal transforms.
### :memo: Checklist
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preview.
URL: https://github.com/sagemath/sage/pull/38093
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
