openapi: document timeline ancestor detach

[koivunej]

The openapi description with the error descriptions:

• 200 is used for "detached or has been detached previously"
• 400 is used for "cannot be detached right now" -- it's an odd thing, but good enough
• 404 is used for tenant or timeline not found
• 409 is used for "can never be detached" (root timeline)
• 500 is used for transient errors (basically ill-defined shutdown errors)
• 503 is used for busy (other tenant ancestor detach underway, pageserver shutdown)

Cc: #6994

[github-actions]

3126 tests run: 2999 passed, 0 failed, 127 skipped (full report)

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Flaky tests (3)

Postgres 16

• test_no_config[application/json]: release

Postgres 15

• test_secondary_background_downloads: release

Postgres 14

• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 31.3% (6414 of 20478 functions)
• lines: 48.0% (49311 of 102655 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
d8e56e7 at 2024-05-22T14:03:51.423Z :recycle:}

[jcsp]

A few comments, no hard blockers

[koivunej]

• test_no_config[application/json]: release

Fix in #7839