Fix documentation for geometric selections with PBC (#2166)

[AkshitBoora]

Fixes #2166

Updates the documentation for geometric selections to correctly describe
how periodic boundary conditions (PBC) are handled.

Changes:

• Clarified that geometric selections use capped distance searches
• Mentioned the use of periodic KD-trees for handling PBC
• Removed outdated explanation about periodic behaviour

This improves the accuracy of the documentation and aligns it with the
current implementation in MDAnalysis.

---

📚 Documentation preview 📚: https://mdanalysis--5305.org.readthedocs.build/en/5305/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.82%. Comparing base (50dc512) to head (87225d6).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5305   +/-   ##
========================================
  Coverage    93.82%   93.82%           
========================================
  Files          182      182           
  Lines        22487    22487           
  Branches      3196     3196           
========================================
  Hits         21099    21099           
  Misses         925      925           
  Partials       463      463           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[jeremyleung521]

Hi @AkshitBoora. We admire your enthusiasm towards improving MDAnalysis, but we won't be reviewing your PRs unless you restore the PR template and answer those sections in detail. This is the same with #5311. Thanks.