[ADD] describing fields in `event.json` file

[JuliusWelzel]

PR Description

As currently implemented, the events.json file being written does not contain information about the two required columns "onset" and "duration" in the .json file.

As those columns are REQURIED, I would propose adding some background information to the .json file every time events are written for people who are not familiar with the structure of the events.tsv files in.REQUIRED.

See https://github.com/mne-tools/mne-bids/issues/1254 for details.

• closes #1254

Merge checklist

Maintainer, please confirm the following before merging. If applicable:

• [ ] All comments are resolved
• [ ] This is not your own PR
• [ ] All CIs are happy
• [ ] PR title starts with [MRG]
• [ ] whats_new.rst is updated
• [ ] New contributors have been added to CITATION.cff
• [ ] PR description includes phrase "closes <#issue-number>"

[welcome[bot]]

Hello! 👋 Thanks for opening your first pull request here! ❤️ We will try to get back to you soon. 🚴🏽‍♂️

[codecov[bot]]

Codecov Report

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

Project coverage is 97.45%. Comparing base (87eea28) to head (3a37466). Report is 25 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1255 +/- ## ========================================== - Coverage 97.61% 97.45% -0.17% ========================================== Files 40 40 Lines 8685 8685 ========================================== - Hits 8478 8464 -14 - Misses 207 221 +14 ```

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[JuliusWelzel]

Hopefully all done, so should be good to go

[welcome[bot]]

🎉 Congrats on merging your first pull request! 🥳 Looking forward to seeing more from you in the future! 💪

[sappelhoff]

Thanks @JuliusWelzel!