create-testnet-data: document in more details subtleties of Conway genesis initialization

[smelc]

Changelog

- description: |
    Give more details regarding creation of Conway genesis in comments (see https://github.com/IntersectMBO/cardano-ledger/issues/4782)
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  - documentation  # change in code docs, haddocks...

Context

Follow-up of #991 (comment)

How to trust this PR

It's only comments

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Self-reviewed the diff