docs: update for new autosquash on automerge

[jonchang]

Update the Homebrew/core maintainer guidelines to talk about the new automatic pull request squashing/amending when BrewTestBot thinks it knows how to do it.

Closes #8093.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

---

[chenrui333]

👍