Docs: Clarify the requirement for signed commits in merge requests
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https://docs.gitlab.com/user/project/repository/push_rules/#require-signed-commits
## Summary
Currently, the documentation doesn't explicitly state how the requirement for signed commits works for merge requests:
* What if the author of the MR doesn't have a GPG signature? Does it only matter that the developer merging the MR has a signature?
* Does it matter that the commits were created through the web interface (single-file editor or **Web IDE**)?
* What if there are more than one commit and from different authors (without signatures)?
* Does it matter whether the commits are squashed?
* Does it matter if the MR author uses community forks?
* How does applying **Code Suggestions** (by a MR author from a developer or vice versa) affect this?
* Any other conditions ...
## Update
An initial attempt to clarify this documentation was made in !209455, but based on feedback, the behavior is complex and varies significantly between GitLab.com and GitLab Self-managed, particularly around web-based commit signing capabilities.
**Current status:** Documentation improvements are on hold pending the rollout of web-based commit signing features, which will affect how these scenarios work.
## References
The source of the questions is https://gitlab.com/gitlab-org/gitlab-services/version.gitlab.com/-/merge_requests/239. In it, I still managed to create an MR (merged) without GPG. I used the **Web IDE**, community forks, and commit squashing. I'm not sure which of these factors mattered.
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