Add YARD docs to SolidusPromotions::Benefit

[mamhoff]

Summary

Add YARD docs for the promotion benefit class.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have localized any and all user-facing strings that I added to the source code.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.34%. Comparing base (06ebabf) to head (092dfd4).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6359   +/-   ##
=======================================
  Coverage   89.34%   89.34%           
=======================================
  Files         961      961           
  Lines       20173    20173           
=======================================
  Hits        18024    18024           
  Misses       2149     2149           

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