docs: document the -sorted extension

[sylvestre]

Fill in the extensions.md stub; -sorted is the one predicate GNU lacks.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.28%. Comparing base (015dc1f) to head (ec1ce75).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #877   +/-   ##
=======================================
  Coverage   92.27%   92.28%           
=======================================
  Files          35       35           
  Lines        7499     7503    +4     
  Branches      390      390           
=======================================
+ Hits         6920     6924    +4     
  Misses        437      437           
  Partials      142      142           

☔ View full report in Codecov by Harness.
📢 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.

[codspeed]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing sylvestre:docs-sorted (ec1ce75) with main (526221e)}

[github-actions]

Commit ec1ce75 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 316 / PASSED: 273 / FAILED: 37 / SKIPPED: 6
  Reference: TOTAL: 317 / PASSED: 274 / FAILED: 37 / SKIPPED: 6

Changes from main branch:
  TOTAL: -1
  PASSED: -1
  FAILED: +0

No result in this run (1) - hung, crashed, or renamed:
  ? gnu/executable (was PASS)