Versions docs: Add note on unstable versions

[lukasbestle]

• 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?

---

The explanation that unstable versions are not acceptable was already in https://docs.brew.sh/Acceptable-Formulae#niche-or-self-submitted-stuff, but not in the Versions docs, which made me believe that unstable versions are fine for versioned formulae (see Homebrew/homebrew-core#50882).

I have added a note to the Versions docs for clarity.

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @lukasbestle!