test fix, doc after gc blocking becoming transient

[koivunej]

• drive-by test fix
• describe the async mutex

[github-actions[bot]]

4968 tests run: 4804 passed, 0 failed, 164 skipped (full report)

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Flaky tests (7)

#### Postgres 17 - `test_tenant_config`: [debug-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9059/10939543135/index.html#suites/4c54991a1bdef2d920f741ae5ea7d1a1/c2847f7122073088/retries) #### Postgres 16 - `test_ondemand_wal_download_in_replication_slot_funcs`: [release-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9059/10939543135/index.html#suites/180444c850d4a41d41eb0a410dc16d84/896a637d385262f2/retries) - `test_subscriber_restart`: [release-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9059/10939543135/index.html#suites/8be0c222d5601535470e7e5978bbfb03/ebea7585350337e8/retries), [release-arm64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9059/10939543135/index.html#suites/8be0c222d5601535470e7e5978bbfb03/b242ed8a8d7f7bcd/retries) #### Postgres 15 - `test_location_conf_churn[1]`: [release-arm64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9059/10939543135/index.html#suites/231aea51ea102d83d3dae52232693454/7faa25f23ea635d8/retries) - `test_subscriber_restart`: [release-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9059/10939543135/index.html#suites/8be0c222d5601535470e7e5978bbfb03/5a6a90f97fb658ed/retries) - `test_pull_timeline[True]`: [release-arm64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9059/10939543135/index.html#suites/82004ab4e3720b47bf78f312dabe7c55/75ff717e1dc4361f/retries)

Code coverage* (full report)

• functions: 31.8% (7415 of 23295 functions)
• lines: 49.9% (59575 of 119387 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
a43a77853355b937a79c57b07a8f05607cf29e6c at 2024-09-19T12:10:49.617Z :recycle:}