Add RBFE equilibration method details

[jthorton]

Fixes #1511, adding a note on the simple equilibration method used in the RBFE protocol.

Checklist

• Added a news entry

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• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[github-actions]

No API break detected ✅

[hannahbaumann]

Thanks @jthorton , lgtm!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.40%. Comparing base (a2f08ac) to head (78bc20d).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1581      +/-   ##
==========================================
- Coverage   95.44%   93.40%   -2.04%     
==========================================
  Files         174      174              
  Lines       14537    14545       +8     
==========================================
- Hits        13875    13586     -289     
- Misses        662      959     +297     

Flag	Coverage Δ
fast-tests	93.40% <ø> (?)
slow-tests	?

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