Add documentation to `Data.Value`

[ana-pantilie]

Pre-submit checklist:

• Branch
  • [ ] Tests are provided (if possible)
  • [ ] Commit sequence broadly makes sense
  • [ ] Key commits have useful messages
  • [ ] Changelog fragments have been written (if appropriate)
  • [ ] Relevant tickets are mentioned in commit messages
  • [ ] Formatting, PNG optimization, etc. are updated
• PR
  • [ ] (For external contributions) Corresponding issue exists and is linked in the description
  • [ ] Targeting master unless this is a cherry-pick backport
  • [ ] Self-reviewed the diff
  • [ ] Useful pull request description
  • [ ] Reviewer requested