Clearer documentation for ExtensionPoint

[jglick]

Amending d4c6a4b as suggested in #11201 (comment).

Testing done

Javadoc generation passes.

Proposed changelog entries

• N/A

Proposed changelog category

/label skip-changelog

Proposed upgrade guidelines

N/A

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
• Proper changelog labels are set so that the changelog can be generated automatically.
• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[daniel-beck]

Thanks!

[daniel-beck]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback. Please see the merge process documentation for more information about the merge process.

/label ready-for-merge