Update TxInsReference haddock to mention public key inputs

[carbolymer]

Changelog

- description: |
    Update TxInsReference haddock to mention public key inputs
# 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

IntersectMBO/cardano-node#6174 (comment)

Adds clarification public key inputs resolved datum-related types.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[Jimbo4350]

This is till too opaque. Imagine coming back to this in a year. Include the reason why this was added and what problem it solves.

[carbolymer]

#855