Improve error messages around getBy with NULLABLE fields

[jkeuhlen]

Before submitting your PR, check that you've:

• Documented new APIs with Haddock markup
• Ran fourmolu on any changed files (restyled will do this for you, so
  accept the suggested changes if it makes them)

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Bumped the version number if there isn't an (unreleased) on the Changelog
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

This PR adds two simple documentation updates stemming from #1607

[parsonsmatt]

Looks great! Do you mind adding a changelog entry for persistent/CHANGELOG.md in an 'unreleased' block with a link to this PR?

[jkeuhlen]

Will do!