Document how and when Dependabot triggers auto rebase

[yeikel]

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What article on docs.github.com is affected?

https://docs.github.com/en/code-security/dependabot/working-with-dependabot/managing-pull-requests-for-dependency-updates#changing-the-rebase-strategy-for-dependabot-pull-requests

What part(s) of the article would you like to see updated?

Dependabot's approach to auto-rebasing pull requests appears non-deterministic or unclear. According to the documentation:

By default, Dependabot automatically rebases pull requests to resolve any conflicts

However, this doesn't always happen immediately. Sometimes, Dependabot triggers a rebase when a pull request is out of date, but other times it doesn't.

Similarly, changes to the target branch may or may not prompt a rebase of existing pull requests, and this behavior seems inconsistent. Manually running a new update job doesn't trigger rebases, it only creates new pull requests. Manually forcing the rebase/recreate commands does work reliably, but it's unclear when it's necessary versus when Dependabot will handle it automatically.

Examples:

yeikel/dependabot-limit-tests#113

The commit yeikel/dependabot-limit-tests@d6b0eff is expected to trigger a rebase to update the existing out of date pull requests. But as of the time of this message that did not happen yet. It may happen eventually, but that's what's confusing

Example out of date pull request:

Example of the current workflows running:

However, here is another similar scenario where the rebases did happen without intervention