Add a comment to coupled boundary condition

[efaulhaber]

Follow-up to #1979.

[github-actions[bot]]

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Code quality

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Testing

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Created with :heart: by the Trixi.jl community.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.16%. Comparing base (0358146) to head (6d5d8db).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1990 +/- ## ======================================= Coverage 96.16% 96.16% ======================================= Files 460 460 Lines 36980 36980 ======================================= Hits 35560 35560 Misses 1420 1420 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1990/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1990/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `96.16% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework#carryforward-flags-in-the-pull-request-comment) to find out more.

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