docs: document the -sorted extension

[sylvestre]

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

[Copilot]

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[codspeed]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

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_{Comparing sylvestre:docs-sorted (ec1ce75) with main (4755e1d)}

[github-actions]

Commit ec1ce75 has test result changes:

bfs testsuite:

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

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