iox-#1293 Document workaround for documentation in internal headers

[elBoberido]

Notes for Reviewer

This PR documents the workaround we used to include the documentation in internal headers which where referenced by a trampoline header.

Pre-Review Checklist for the PR Author

1. Code follows the coding style of CONTRIBUTING.md
2. Tests follow the best practice for testing
3. Changelog updated in the unreleased section including API breaking changes
4. Branch follows the naming format (iox-123-this-is-a-branch)
5. Commits messages are according to this guideline
   • Commit messages have the issue ID (iox-#123 commit text)
   • Commit author matches Eclipse Contributor Agreement (and ECA is signed)
6. Update the PR title
   • Follow the same conventions as for commit messages
   • Link to the relevant issue
7. Relevant issues are linked
8. Add sensible notes for the reviewer
9. All checks have passed (except task-list-completed)
10. Assign PR to reviewer

Checklist for the PR Reviewer

• Consider a second reviewer for complex new features or larger refactorings
• Commits are properly organized and messages are according to the guideline
• Code according to our coding style and naming conventions
• Unit tests have been written for new behavior
• Public API changes are documented via doxygen
• Copyright owner are updated in the changed files
• All touched (C/C++) source code files from iceoryx_hoofs have been added to ./clang-tidy-diff-scans.txt
• PR title describes the changes

Post-review Checklist for the PR Author

1. All open points are addressed and tracked via issues

References

• Closes Don't exclude implementations of public classes in Doxygen #1293

[elfenpiff]

Minor typos