docs: clarify prCreation=not-pending fallback and no-check caveat

[RahulGautamSingh]

Changes

Documents the prCreation=not-pending behavior discussed in #44161:

• Correct the fallback timeout from "24 hours" to the actual prNotPendingHours default of 25 hours, and link the option (both in the prCreation section and the automergeType=branch backup-PR note).
• Note that the fallback is measured from the branch's latest commit, so rebases/re-locks reset the timer.
• Warn that not-pending can stall PR creation when branches have no external checks (e.g. CI only on pull_request), and point to internalChecksAsSuccess=true / prCreation=immediate as fixes.
• Add the same caveat to the "Suppress branch/PR creation for X days" example.

Context

Please select one of the following:

• This closes an existing Issue, Closes: #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

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• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation). Docs drafted with Claude (Opus 4.8); figures verified against lib/config/options/index.ts and lib/workers/repository/update/pr/index.ts.
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Documentation (please check one with an [x])

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The public repository:

[klippx]

Only one suggestion:

In the original issue I mentioned that

The docs at Suppress branch/PR creation for x days recommend this combo to hold updates for a few days:

 {
   "minimumReleaseAge": "3 days",
   "internalChecksFilter": "strict",
   "prCreation": "not-pending"
 }

I think that maybe this needs to be updated (adding "internalChecksAsSuccess": true ?)- or at least something need to mentioned immediately close to this "official recommendation", because it was this mis-recommendation that caused all our suffering.

[RahulGautamSingh]

or at least something need to mentioned immediately close to this "official recommendation"

Already done, pleas recheck the changes/additions in the PR