Fix doc references for nullary expressions.

<!-- 
Thanks for contributing a merge request!

We recommend that first-time contributors read our [contribution guidelines](https://eigen.tuxfamily.org/index.php?title=Contributing_to_Eigen).

Before submitting the MR, please complete the following checks:
- Create one PR per feature or bugfix,
- Run the test suite to verify your changes.
  See our [test guidelines](https://eigen.tuxfamily.org/index.php?title=Tests).
- Add tests to cover the bug addressed or any new feature.
- Document new features.  If it is a substantial change, add it to the [Changelog](https://gitlab.com/libeigen/eigen/-/blob/master/CHANGELOG.md).
- Leave the following box checked when submitting: `Allow commits from members who can merge to the target branch`.
  This allows us to rebase and merge your change.

Note that we are a team of volunteers; we appreciate your patience during the review process.
-->

### Description
<!--Please explain your changes.-->

Fix doc references for nullary expressions.

### Reference issue
<!--
You can link to a specific issue using the gitlab syntax #<issue number>. 
If the MR fixes an issue, write "Fixes #<issue number>" to have the issue automatically closed on merge.
-->

Fixes #2997.

### Additional information
<!--Any additional information you think is important.-->

Closes #2997

See merge request libeigen/eigen!2054