docs: mention being in @beta

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 📝 Documentation: Mention @beta tag in docs #1903
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

🎁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.40%. Comparing base (67c686c) to head (4d1d6e6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1904   +/-   ##
=======================================
  Coverage   95.40%   95.40%           
=======================================
  Files          86       86           
  Lines        3523     3523           
  Branches      201      201           
=======================================
  Hits         3361     3361           
  Misses        162      162           

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