Minor prettyprinter fixes

[newhoggy]

Changelog

- description: |
    `Error` instance for `FileError` instead of `Pretty`
    Make `prettyToText` return strict `Text` and add `prettyToLazyText`
    Export `Doc`
# 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
  # - improvement    # QoL changes e.g. refactoring
  # - 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

Additional context for the PR goes here. If the PR fixes a particular issue please provide a link to the issue.

How to trust this PR

Highlight important bits of the PR that will make the review faster. If there are commands the reviewer can run to observe the new behavior, describe them.

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

[carbolymer]

Is Ann a default Doc annotation for api, or what's its purpose? Can you add a short comment explaining what's Ann?

[newhoggy]

Yep. It is behind a type alias so we can change the implementation if needed.

[newhoggy]

Added comment.