📝 Docs: Add issue/PR description stipulations to AI contribution policy

### Documentation Request Checklist - [x] I have read and agree to Mocha's [Code of Conduct](https://github.com/mochajs/mocha/blob/main/.github/CODE_OF_CONDUCT.md) and [Contributing Guidelines](https://github.com/mochajs/mocha/blob/main/.github/CONTRIBUTING.md) - [x] I have searched for [related issues](https://github.com/mochajs/mocha/issues?q=is%3Aissue) and [issues with the `faq` label](https://github.com/mochajs/mocha/issues?utf8=%E2%9C%93&q=is%3Aissue%20label%3Afaq%20), but none matched my issue. - [x] I want to provide a PR to resolve this ### Overview https://github.com/mochajs/mocha/blob/a2e2c497d93bd6d518bc8a61e486993296e2da76/.github/CONTRIBUTING.md#-ai-generated-code only says these three high-level points right now: * All automated tests must pass * Manual review is required * You are responsible It doesn't indicate that we don't want clearly LLM-generated descriptions. I think we should add that in. If a PR has the annoyingly super verbose description, that's a clear indicator it's not piloted by a human who's reviewing things to the level we'd want them to be. ### Additional Info Prior art in https://typescript-eslint.io/contributing/ai-policy: > Avoid AI-generated content in written, non-code contributions (issues/comments/PR descriptions, etc.). AI-generated PR descriptions in particular tend to just be nearly-verbatim descriptions of the code diff, which add no value to the PR. Instead, we ask that you summarize the PR succinctly in your own words, relying on your own understanding of the code.