curl_url_set.md: expand the CURLU_NO_AUTHORITY description

[bagder]

No description provided.

[Copilot]

Pull request overview

Expands documentation for CURLU_NO_AUTHORITY, clarifying its behavior for hostless URLs using known schemes.

Changes:

• Adds known-scheme examples.
• Explains default parse-error behavior.

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[testclutch]

Analysis of PR #22515 at 366227d2:

Test 2100 failed, but it has been 1.4% flaky lately, so it's probably NOT a fault of the PR. Note that this test has failed in 4 different CI jobs (the link just goes to one of them).

Test ../../tests/http/test_21_resolve.py::TestResolve::test_21_07_dnsd_a[DoH] failed, which has NOT been flaky recently, so there could be a real issue in this PR. Note that this test has failed in 2 different CI jobs (the link just goes to one of them).

Test ../../tests/http/test_21_resolve.py::TestResolve::test_21_08_dnsd_aaaa[DoH] failed, which has NOT been flaky recently, so there could be a real issue in this PR. Note that this test has failed in 2 different CI jobs (the link just goes to one of them).

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